Fastpass is at certain (popular) attractions at the park. You put your pass into this machine like thing and a little ticket comes out with a time on it. You go back to that attraction at the time stated on the ticket and you wait in another line. We used it several times, luckily it wasn't that crowded when we were there and onl had to do it on wkends.
There is no extra charge, you just use your regular pass to do it.