The Protagonist is the principal character of a story, usually the hero, but not always. Beyond any and all other traits, whether they are a Villain Protagonist, Hero Protagonist, or simply an Innocent Bystander, and whatever the story's POV may be, you can generally tell who the protagonist is because the story is about them. This is present in pretty much all fiction, making it an Omnipresent Trope. As a result, this page only defines the term. While the Protagonist is generally a Hero, not all Protagonists are Heroes.
Compare and contrast The Antagonist, who is equally important because they oppose the actions of the protagonist, although there are stories without an antagonist. See also Deuteragonist, another character the story is about, but to a lesser extent than the protagonist; in other words, a secondary main character.
It has a few Sub Tropes, such as Decoy Protagonist, a character who appears to be the protagonist but turns out not to be, Rogue Protagonist, a protagonist becoming the antagonist in a sequel, and Supporting Protagonist. This trope tends to run on Survivorship Bias, since we care more about the protagonist than other characters.