A buff is a skill or status effect that enhances a players abilities so they can fight or move more efficiently. Most famous for such skills are the Assist class, for they possess the ability to cast buffing spells upon other characters. Note that in the Player-versus-Player system, the Resurrection skill is considered a buff.
The opposite of a buff is a debuff, which is cast on another player or a monster and makes them somehow weaker. Unlike buffs, which only Assists can cast on other players, nearly every class has debuffs they can use.