A buff is generally a non-permanent attribute added to a unit, affecting it directly or indirectly for a period of time in a positive manner. Such an effect is the Barbarian Battle Cry ability to strengthen other characters. It applies to a single target, or targets in an Area of Effect but it retain this "buff" even when they move around.
The direct opposite of a buff is a debuff, that works similarly, but has direct/indirect negative effects on the recipient.