Okay, so you buy an Express Pass. The ones you purchase in the park are good for 1 time on every attraction.
You can purchase the Express Pass online, at the ticket windows, Guest Services, or various Merchandise locations throughout both parks.
They will give you a ticket and it was say "Islands Express", "Studio Express" or "Combo Express". Islands is only good at Islands of Adventure, Studios is only good at Universal Studios, and Combo is good at both parks.
Let's say you want to go on Spiderman but the wait is 60 minutes. If you use your Express ticket, the attendant scans the barcode with a handheld scanner and sends you into the Express Line, which is a shorter line onto the attraction. You still may wait a bit, but 15 minutes is what they try to keep the Express Lines at.
Lets say you come out and want to do it again. You go to the Express Entrance again, the attendant scans your ticket, it will read as invalid because you've already used it. It's only good once on every attraction. It's like buying a FastPass for every ride.
So every attraction takes Express Pass except Pteranadon Flyers in Jurassic Park. No times to come back, it's just whenever you want to use it.