Hop Up is the system that guide bb's from the magazine to the barrel, and provide back-spin on the bb.
This is adjustable, which allows for a more accurate and longer flight trajectory.
Nearly all AEG's use the same type hop up rubber. Almost all AEG hop up rubbers fit on virutally all AEG barrels. Exceptions on this are special barrels, like the Modify Hybrid Barrels, Sniper Rifle Barrels and Gas Guns.