This is a writing or directorial choice that involves the use of demonstrative techniques, rather than blatant or thinly-veiled narration, to establish narrative elements.
For example, say Alice is a badass:
- To show that Alice is a badass, she would spend the entire book doing indisputably badass things. More pertinently, the book would go into detail: for instance, the work could begin with a Batman Cold Open where she takes on six mooks without breaking a sweat. In these circumstances, we don't have to be told she's badass; we can see it for ourselves.
- To tell that she is a badass, the narrator, Alice herself and/or other characters around her would merely state that fact. For instance, they might report on previous incidents that have happened in the past and/or "offscreen" while the other characters were busy. Or maybe there'll be no support for the statement whatsoever, but that's unlikely ("Hey, did you hear about the badass things Alice did the other day?" "No, I didn't." "Well, they sure were badass!" *crickets*).
If you're using a story structure or Point of View that doesn't include a narrator (such as limited third-person, in which you only see into the head of one character), showing is usually a better idea, if only because having a narrator suddenly show up just to tell this stuff would break the reader's Willing Suspension of Disbelief. It's even more important in a visual medium, since people don't tend to say precisely what they're thinking or how they feel about it for a hypothetical audience's benefit; watching two characters discuss the details of something they both already know rather than making economical use of a flashback to when one or both didn't know is extremely poor storytelling.
This also relates to sentence-by-sentence writing decisions that have more to do with an author's language and word choice than anything else. In general, something happens in every sentence written. Is the author merely stating those events, or describing them? "Alice was angry and upset over Bob's death" is the telling version of "As her husband slumped to the floor, with blood gushing from his throat, Alice's heart raced and she choked back tears." One of these two sentences has slightly more dramatic power, and it's for reasons of impact that showing is generally advocated over telling.
Now this line is sometimes quoted as an absolute gospel truth, which is not really true. It's certainly a good habit to get into (particularly in character writing; nobody likes being told what they're supposed to think of someone), but it's not an ironclad rule, and knowing when to break it to quickly explain minor details is a major aspect of learning to write. One of the best times to Tell something instead of Show it is when you want to summarize lots of events—the written equivalent of a Time Passes Montage. Some times, one might Show so much that it becomes Too Much Information.
It should also be noted that action is not the same as showing, and dialog is not the same as telling. If characters are having a conversation, things can be revealed, but the way they are revealed, and how others react, can be a form of showing. Conversely we could see characters doing their job, but nothing else is revealed, so while we are shown characters have employment, it would not show much else about the character.
An extension of the concept in interactive media like Video Games is "play, don't show." Rather than the player being told that the Dragon Lord killed your ninja clan and dishonored you by defeating you in a duel or being shown a movie sequence, the player is allowed to act out the journey to the Doomed Hometown and fight a Hopeless Boss Fight against the far more powerful foe prior to the game proper.
- As You Know: As you know, this is when a troper like you recognizes the act of characters giving out exposition nobody in the scene would need.
- Info Dump: Infodumping (that is, information + dumping) is a type of Exposition that is particularly sesquipedalian. Although it can be done in a way that is unintrusive or entertaining, most infodumps are obvious, intrusive, patronizing, and sometimes downright boring. Specifically, if the premise of your story is laughably ridiculous, an infodump will call attention to the fact. This infodump, for instance. The word 'infodump' is often used as a pejorative.
- Just a Face and a Caption: Images for tropes should show the trope being used, rather than just have the caption tell how the trope is used in the image.
- That Makes Me Feel Angry: It makes me sad when writers resort to just having their characters say what they feel in so many words.
- Exposition: Dialog informs other characters, and also the audience, of key information.
- Explaining Your Power to the Enemy: When a character's power is spelled out by that character, rather than made clear through visual representation.
- And That's Terrible: When the narrator reminds the audience, in case they missed it, that what the villain did was terrible. Which is a bad device and shouldn't be used.
- Narrating the Obvious: When a story shows and tells.
- Said Bookism: "In which the nature and intention of dialogue is told by the dialogue tags as well as by the dialogue itself," the TV Trope page vocalized speechlessly with passion and vigor.
Telling that contradicts shown behavior or evidence:
- Character Shilling: "Wow," said Alice, "Bad Bob is the most amazing guy in the world, isn't he?" "He sounds wonderful; I can't wait to meet him in person." said Carol.
- Continue Your Mission, Dammit!: There isn't much time left because people keep telling you there isn't much time left. You have 35 seconds to finish this page.
- Designated Hero: When we've got nothing but the narrative's word for it that the fellow the book follows is, in fact, the good guy.
- Designated Love Interest: When we've got nothing but the narrative's word for it that the fellow the book follows is, in fact, romantically attached to the heroine.
- Designated Villain: Likewise, the character in question has never done anything especially evil, but the narrative leaves no room for doubt. Often paired with Designated Hero, though you can have one without the other.
- Esoteric Happy Ending: When the author thinks the story ends on a happy note.
- Faux Action Girl: We're told she's a badass, but it certainly doesn't look that way.
- Hollywood Homely: Casting an attractive actor or actress to play someone who's supposed to be bland looking or even downright ugly.
- Hollywood Pudgy: Alice is of ideal or slightly below ideal weight, has broad shoulders, and has round cheeks. Characters act as though Alice is fat.
- Informed Attribute: We are told that Alice is smart/funny/ugly/pretty/a vampire, but we never see any evidence for this. Subsets include:
- Informed Ability: "...and Alice, who could turn into a pink butterfly on a whim, dragged herself out of bed and drove to work."
- Informed Attractiveness: Alice is described as drop-dead gorgeous in story, but the audience doesn't really react to her that way.
- Informed Deformity: "Alice and Bob might both have been stick figures, but by God, he was hideous."
- Informed Flaw: You are blind.
- Informed Judaism: "Oy vey", said Alice, munching on a bagel. "Didn't you guys ever notice I don't go to church on Sundays?" Bob replied, "I noticed you don't go anywhere on Saturdays, either!"
- Informed Loner: "I don't like being around others," said Alice to Bob and 20 of their very best friends. On being told this, nobody left the room.
- Informed Poverty: Alice is said to be poor yet she lives in one big house.
- Informed Wrongness: Bob told Alice that the moon orbits the Earth, ignoring the clear evidence she'd presented that everything in the universe orbited her. Well, we can't all be smart like Alice.
- Offscreen Moment of Awesome: While you were away, the hero defeated the villain in a truly epic battle; you'll just have to take our word for it because it was awesome.
- Offstage Villainy: The villain is said to have done something malicious, but we do not get to see it happen to understand how evil he/she really is.
- The Pirates Who Don't Do Anything: Have a job which would involve distinctly non-heroic behavior for your hero? Don't worry, just don't have them do it. It's not like killing people is an important part of being an assassin anyway.
Showing instead of playing:
- Cutscene Power to the Max: Your character is significantly more able when you're not playing as them.
- Cutscene Incompetence: Your character is significantly less able when you're not playing as them.
- Gameplay and Story Segregation: The game mechanics don't work the same way the storyline does, or the story doesn't match the way the player is allowed to behave. This often comes across as the game simply ignoring whatever the player is doing to tell a fixed, immutable storyline.
Playing instead of showing:
- Degraded Boss: A way of playing out Villain Decay by having the boss return as a normal enemy or Mini-Boss.
- Instructive Level Design: Objects in the levels are assembled such that the designers don't need to say anything to you.
- Malevolent Architecture: Environments may be twisted or something in such a way as to make you anxious when moving through them.
- And Some Other Stuff: We're told that the characters made something dangerous, but the ingredients aren't shown, to prevent idiots from trying this at home.
- Bolivian Army Ending: When it's already obvious that the protagonists have met their match.
- Discretion Shot: Writers imply that something violent or sexual happened without showing it to avoid censorship.
- Informed Conversation: The "would otherwise be repeating what the audience has seen already" and "distill the plot" variants.
- Great Offscreen War: Not every writer can convincingly depict a war. Especially if they have no experience with the subject.
- Noodle Incident: Writers don't even tell the details, to let imaginations fill in the gaps.
- Noodle Implements: Stating items used, but not how, to let imaginations fill in the gaps.
- Nothing Is Scarier: When the writers use our imagination to make us fear.
- Take Our Word for It: Writers describe something they can't possibly live up to by showing it, so they just tell us what it's like, and let our imaginations fill in the gaps.
- Time Skip: No one wants to read or watch through a lengthy sequence of events where nothing particularly interesting, exciting or relevant to the plot happens. Under such circumstances, it's quite acceptable to just jump from one relevant bit to another relevant bit and tell your reader / viewer that it's "ten years later" or "it took him fifteen minutes to get to the shop" and leave it at that without showing what happened during that time in unnecessary detail.
- You Cannot Grasp the True Form: Otherworldly entities and places are impossible by definition to actually pull off.
- You Do Not Want To Know: Some secrets are best left unrevealed.
Showing that slows the narrative:
- Description Porn: Description that goes into so much detail that the reader will soon shout "Get on with the story already!"
- Continuity Porn: An abundance of references to previous installments, which risks a Continuity Lockout or, at least, takes up room that could be used for new storyline;
- Costume Porn: Costumes that are detailed far beyond what is needed to characterize the wearer can distract readers from what they're actually doing.
- Design Student's Orgasm: Animation that is so detailed that the viewer will forget what the story is about again.
- Scenery Porn: Extreme amount of detail put into designing scenery that the audience will either ignore or get annoyed at the real action blocking.
- Purple Prose: Pretentious, extravagant wordplay that will make readers want to skim through for fear of pages and pages of mundane description.
- Mills and Boon Prose: Overly elaborate descriptions of sensual encounters that enrapture readers away from the real action.