The enhanced fastpass allows you to collect a fastpass for each attraction simultaneously. It essentially unlinks all the attractions. It's not really unlimited, because the normal rules apply for collecting the next fastpass for a specific attraction. For example, at 10:00 AM. you may collect a fastpass for Space Mountain with a return time of 12:00-1:00 Noon. You will not be able to collect another FP for Space Mountain until noon, but you can go ahead and collect a FP for Buzz, BTMRR, Splash, Indiana, etc.
The Enhanced FP is only available when you book a package through AAA that includes a multi-day parkhopper and stay at one of the three Disney resorts. It is a fabulous perk, especially during busing periods for those who like to ride lots of E-rides.