Buffs refer to a temporary beneficial spell or effect placed on a player. Buffs can range in effect from simple personal shields to buffs which augment almost every category of stats a given player may have for their race or class.
Buff can also be used as a verb, i.e. "to buff" means to apply buffs.
Buff is also used to represent a change to a class or ability that makes it more effective or useful. It is the opposite of a nerf.