The critical multiplier indicates how many times damage is rolled in the case of a critical hit with a given weapon. (Damage is not multiplied, but rolled multiple times.) It is usually indicated after the threat range for a weapon, as in "19-20/x2" indicating that damage is rolled twice. When the threat range is 20, it is sometimes omitted, and just the critical multiplier is used to indicate the critical threat of a weapon (e.g. "x3").
The increased multiplier feat is the only way to change the critical multiplier of a weapon in Neverwinter Nights.
When dual-wielding, a bug in the game engine causes non-physical damage dealt by the off-hand weapon to use the critical multiplier of the main hand weapon, rather than of the off-hand weapon.