I am not an expert at this, and there certainly are more knowledgeable people on this board, but I will take a stab at explaining it. This is my understanding:
FOTL is an acronym for Front Of The Line. Each ride, except for P. Flyers in IOA has two lines to enter the queuing area. The Express line loads faster than the regular queue due to it being much shorter. So, it really isn't a literal Front of the Line, just that you get to bypass the regular queue and get on the ride considerably quicker than the regular line. For instance, if a wait time on a ride is 45 - 60minutes, the Express line wait is usually 5-20 minutes. In all our trips, waiting 20 minutes in an Express line would be considered an oddity and would cause much spoiled whining and grousing.
There are three ways you can access the Express Line.
1) With your onsite hotel (Portofino, Hard Rock or Royal Pacific) room key - this gives you unlimited access to the express line of every ride, any park, anytime of day. There are no limits on how many times you can ride, nor are there specific times you have to ride. FOTL is available from the time you check in to Park closing of the day you check out. There are a few exceptions during the busiest of times, where Universal has limited the FOTL to one time per ride before 3:00pm and then unlimited (from 3:00-10pm) use of FOTL after that.
2) Purchasing the Express Plus Pass - this is a pass that you can purchase, in addition to your park tickets, that allows you a one time use of each express line of every ride. However, each pass is only good for the park you purchase it in and for the day that you purchase it. And each member of your group will need to purchase their own passes if you want to all ride the same rides.
3) Express Passes from the machines - These are free passes that are much like Disney's fast passes. There are kiosks located near each ride. You scan in your park ticket and then will be offered a choice of 2 return times. Choose the time you want to return to the ride and you will be issued a paper Express Pass. However, like the fast pass, there are only a certain amount of Express Passes issued for each ride per day. So, during busy times, there is the possibility that the popular rides might run out of passes by mid-morning.
Most often, when somebody mentions FOTL, they are referring to the Hotel privileges of full time access to the Express line. The other two options are generally referred to as the Express Plus and the Express passes.
Hope this makes sense,