What does Captain America symbolize?It should be an easy answer to the question. Captain America symbolizes everything that America has been and could be. In the movie, all of this is revealed as to what he symbolized when he was "misused" on the home front. As America declined, Captain America has become an uneasy symbol.
Marvel wrote stories for him that were relevant in the 1960s, but in the 2000's and later, the ideals he has comes from a different time. What he stands for: American bravery, American ideals, and American Exceptionalism; is not taught in schools anymore. What academics focus on is America's sins. The Winter Soldier (Bucky Barnes) represents the new America far better than Captain America. But Captain America represents what America can be far better than any superhero in fiction. Including Superman.
This is a very good question, and probably a good-sized book could be written on this topic that would include references to a wide range of topics such as American Exceptionalism, American Idealism, the American nationalistic fervor prominent during WWII and afterward (when Captain America was created), and many stereotypical elements of American culture.
I am not the person to write that book.
But to me, Captain America symbolizes the ideals of American "goodness". The ideal of a good-hearted American man that is genuinely concerned about acting with nobility and justice and doing right by his fellows, is internally righteous-minded (not in a religious sense) and just generally does what he knows to be right in any situation, even if doing so is painful or dangerous.