While out fighting the evils of the land you may need to repair your equipment.
This wonderful glowing stuff will repair up to 100 Durability to the item in your right hand, per use. Crystal items can not be repaired with Repair Powder.
The Undead Male Merchant is the only source of Repair Powder in the game. He will sell it to you for for 500 Souls a bag.
The advantage of Repair Powder over the Repairbox is Repair Powder can be used at any time. A Repairbox can only be used at a Bonfire. This makes Repair Powder very useful for online games where you may not want, or may not be able to return to a Bonfire.