A hero is someone, usually a male, that exceeds what a "regular" person can do. Most (super) heroes are known for their super powers like their strength, flying abilities, x-ray vision, and what have you. That at least is a child's view of what a hero can be, the prefix "super" is automatically added. Children do grow to realize that just because a man isn't wearing neon and unusually tight clothing doesn't he mean he can't be a hero.
Luke Skywalker, Indiana Jones, and Frodo Baggins are obviously known for being heroes. They have all set out on journeys searching to save, steal, or return a precious something or other. They're not heroes because they can fly around, although seeing a hovering hobbit could be entertaining, but because they succeeded in what they sought to do and didn't give up regardless anything else. A hero is determine, strong (mentally or physically), and above all they help people, although it's not usually from giant monsters (unless we're talking about Star Wars).
Males are viewed as heroes more than women because they look more like heroes. They have those toned muscles, are bigger, and are over all masculine. Men, for most of history, have been viewed as stronger due to only their appearance. This is a total misjudgment that society often makes because although men may not agree women usually have more brain power and are mentally stronger.