Repairing is a skill that complements the crafting skills of Blacksmithing and Woodcrafting. If you can create a weapon or a piece of armour, you can also repair it. To repair items made with blacksmithing, you need to be at a forge; other items can be repaired anywhere.

You don't have a level in repairing, and it doesn't count towards levelling your blacksmithing or woodcrafting, but at least you never fail.