From Vanguard Wiki
A buff is a spell or ability which enhances the player character on whom the buff has been cast. Buffs usually wear off (fade) after a set duration, but may be voluntarily removed by the recipient. It may also be possible for the buff to be removed with a dispel. The term most typically refers to a spell designed to increase the statistics of a character, although generally any beneficial qualifies. Some players refer to only stat-based enhancements as buffs, using other terms (such as haste or crack) to refer to speed or regeneration enhancements.
Before important battles, most groups and raids will "buff up" - that is, ensure that all members of the party or raid have the beneficial spells they need.
The opposite of a buff is a debuff, that is a spell designed to weaken an opponent. Debuffs are usually cast by player characters on mobs or on other players in PvP.