It's a pass to return to the ride at a predetermined time .It cuts out the need to wait in the stand-by line. When you get a Fastpass it will have some info on it
1st return time 10:40-11:40 the 11:40 for most rides is not a hard time
then a when you can get another fastpass time (this one lets you get a new one for either the same or a different ride.
If used correctly you can get many things done with very little waiting in lines.