Off hand is a secondary weapon slot, used to wield one-handed weapons or off-hand only weapons. All professions have the ability to equip at least one type of one-handed weapon or off-hand only weapon in their off-hand weapon slot. Using an off-hand weapon will grant the last two weapon skills based on the weapon equipped. If no off hand weapon is equipped in a weapon set, but is equipped in the other set, both weapon sets will use the same off hand weapon.
For new characters, wielding an off hand weapon first becomes possible at level 6 but only unlocks one of its skills. The second skill of the weapon becomes available at level 8.