The off-hand slot is an inventory slot used by off-hand weapons and all non-weapon items that are held in a hand, such as a shield or torch. The item in this slot is unequipped if a two-handed weapon is placed in the main hand slot, and placing an item in this slot will cause a two-handed weapon to be unequipped (if there is one in the main hand slot).
Visually, items in this slot are depicted in the character's left hand (or on the left arm, in the case of shields).