Ability score loss occurs when certain attacks or spells reduce one or more of an opponent's ability scores. By Dungeons & Dragons standards, temporary ability score loss is called ability damage, while permanent loss is called ability drain, but the nomenclature in Neverwinter Nights is more relaxed (in part, perhaps, because even "permanent" ability score loss does not persist into new modules).
Neverwinter Nights does not allow ability scores to drop below 3.
Ability scores can be restored by the "temple healing" available in many modules (including the official campaigns), as well as by the restorative spells (e.g. lesser restoration, remove disease). Resting cures ability score loss that is not supernatural.
If a spellcaster's primary ability is reduced, the caster may lose spell slots (both the bonus slots for high ability scores and slots whose level exceeds the modified score minus ten). These slots are regained if the ability score returns to normal, but they will be empty (no longer assigned spells). For spontaneous casters, this can be a more significant problem as when they lose bonus slots, they always lose readied slots. Thus a spontaneous caster who repeatedly lowers than raises ability scores can potentially lose all spells for the day through this repeated loss of bonus slots.