In real life, there is no universal plot structure that is common to all fiction. But when you read a lot of books, you will recognize certain elements that appear and reappear in several works of fiction – simply because books generally deal with people, and people act in relation to other people and have similar problems and lives insofar as they all love and hate and survive and die and so on. If you collect these similarities and try to join them together, something like a common plot structure appears.
No intelligent person would claim that all books follow the same structure, but since human beings seem to prefer a simplified version of the world to the real world, books describing this simplified common plot structure and "finding" it (i.e. finding parts of it and projecting the missing rest and ignoring any deviations) are very popular, especially among those that have no clue or are gifted with only little brain power or find it unbearable that the world is not kind and not comprehensible.
"Protagonist" (literally the main or primary actor, besides the "Deuteragonist" and "Tritagonist", the second and thrid actors in classical Greek theatre) is the name given to one of these structural elements: the person, whose story is being told. In another traditon, which does not derive from philology and the study of ancient drama theory but from ethnology and the study of magic and religion, the name for this person is the "hero".
A "hero" in this sense is not a hero in the sense of a person performing heoric feats, but simply the person whose life is disrupted by a call to adventure. The hero is not up against an antagonist, but against his own need to grow and transform. Unlike the hero, who only wants to return home and live the life he has always lived, but must change to preserve that life and thereby changes himself and it, the protagonist has an outward goal, he wants away from the life he as always lived, and the antagonist is the force or person that attempts to keep him from getting there. So we have two traditions of the simplification of literature, and they propose completely differing views of what goes on in fiction, and yet there are people who are not satisfied with that and try to merge these two structures into one even more simple one. They identify the antagonist with the hero's own inner obstacles to growth, but, as I said earlier, this identification (and all others) are caused by a need for simplification that has no correspondence in a complex reality.
In short, the hero is one way of simplifying literary diversity, the protagonist is another, and both are not true in the sense that all literature has a hero or antagonist.