One character explains something to another. That is, if he just flat-out explained it with words, it would be boring. The audience will be much more thrilled if they aren't told anything until they SEE it themselves. So the story has the talking character unnecessarily babble that "it can't be explained"/"It's so hard to explain"/"I don't know how to say it", even though it should be easy to put it in words. Examples:
- Morpheus: "No one can be told what the Matrix is. They need to be shown." Nonsense, of course you can explain it to people. Just say "Reality is a simulation made by robots.." etc. In this particular case it's somewhat justified, in that he probably meant you wouldn't believe it unless you were shown.
- In the beginning of the horror film "The Ring", one girl tells the other how she saw the cursed movie. She says "The tape didn't contain the film we wanted to watch but...something else." Why couldn't she explain what that "something else" is? Why didn't she just say "It was weird black-and-white pictures like a floating upside-down chair and a ladder falling over and a creepy girl and stuff"? Oh, so that the audience would only get to see it once the tape was actually shown to them.
- I'm sure there are plenty of other examples I missed which had stuff along the lines of:
Hello, Unknown Troper. You'll need to get known to lend a hand here.