The Protagonist is the principal character of a story. Many are Hero Protagonists but not all of them are. Beyond any and all other traits, whether they are a Villain Protagonist, The Hero, 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 him or her. This is present in the vast majority of all fiction, marking it as one of the rare Omnipresent Tropes.
Compare and contrast The Antagonist, who is equally important because they oppose the actions of the Protagonist. Also see Deuteragonist, the second character whom the story is about, but to a lesser extent than the Protagonist; in other words, a secondary Main Character.