It's Universal's answer to Disney's FastPass with a big improvement. Onsite guests get FOTL to each attraction without having to feed their admission ticket to a machine and then returning at the time that the machine says.
The best reason to stay onsite. You have access to all rides, all day at both parks by just flashing your hotel room key which btw also gives you Priority Seating at most restaurants.
Think of it as Dsiney's Fastpass on steroids. You just show your key card at each attraction and you are allowed in the express line. We never had more than a 15 minute wait. It is unlimited so if you have a favorite ride you can ride it over and over again if you want to.