Fast passes are free.
Basically, it's a return "ticket" back to the ride at a specific time so you don't have to stand in line for 2 hours. The idea is this: Either you go to a ride, stand in line, wait time 45 minutes, and then do the ride for 5 minutes, OR, go to ride, grap a Fast Pass from the electronic machine, tells you to return in 2 hours, go do something else, come back to ride, go in Fast Pass line, all the way to the front of ride, and enjoy your 5 minute ride without waiting/wasting time in line.
It's a great concept and it works. Trust me, it will be your best friend while you are there. It's really the only way to get your money's worth out of the parks.