Buff is a slang term for a spell or effect that improves or protects a character. For example, casting enhancing spells likes bull's strength or spell resistance is called "buffing."
Buff can also refer to module creator or another developer changing the rules to enhance something. For example, changing the spell bull's strength to give +8 ability points instead of +4 might be referred to as "buffing" it -- the opposite of nerf.