It's the Italian equivalent for "Luke," but lots of people confuse it for a girls name because it ends with "a" (typically A-ending names are female and O-ending names are male).
In truth I always tell people it was named after the Brand New song, but the *real* inspiration was Luca Brasi from "The Godfather." I was watching it with my wife one day (I think we may have only been dating at the time, it was forever ago) and mentioned that I really like that name and she agreed. And now here we are, with a son named Luca.
(And to be fair, I think the Brand New song title is a reference directly to that character, so it's kind of a secondhand inspiration by association, in a way.)