Effort Points/Gear Points.
It creates a ratio system to determine loot priority, wherein you're awarded EP for various tasks and given GP when you get loot; your EP divided by your GP is your Priority Rank, and generally loot boils down to whoever is highest on PR among all the people who want the piece.
The layman's result is that the longer you go without receiving loot, the more likely you are to receive some, as everyone else around you is getting their PR adjusted down (thanks to the GP costs) while yours is only going up with every boss killed.
It also prevents people from sitting on longstanding points by causing your total accumulation to decay over time, keeping your ratio in check so newbies don't come along and have zero chance at gear.
XI Wiki



