This skill grants the ability to repair tools using iron blocks. To repair an item you require whatever resource that is used to make the tool plus an iron block. You can repair by right clicking with the tool in hand twice to repair an item. Note, this can downgrade enchantments.
Perks for leveling up repair:
Diamond Repair: Passive ability to allow a person to repair diamond tools and armor. This requires level 50 repair.
Repair Mastery: Adds extra durability multiplier based off of this table: (Table is from mcmmo wiki) the multiplier is .2% per level of repair added onto the base durability restored.
Arcane Forging: This gives the chance to keep enchantments, downgrade enchantments, or lose enchantments all together. The rank is increased every 125 repair levels. The chance to keep enchantments goes up by 10% per rank of this ability up to a total chance of 80% at 1000 repair. Well, the chance of downgrading/loosing enchantments all together go down per rank.