An aptitude is something you're good at. A rock star might have an aptitude for energizing an audience, or for trashing his hotel room.
When you have a talent or do something well, people say that you have an aptitude for it. But you don't have to be born with an aptitude; you can also acquire your abilities. You are probably already well acquainted with this word, since the zillions of standardized tests you have taken in school are usually called aptitude tests.