Durability is a property of some equipment and weapons that determines how much damage it can take before becoming unusable. This means it will not participate in your battles. A piece of equipment of 0 durability will not provide you any defensive or offensive stats.
Equipment that has durability can be repaired at any time using a weapon or armor repair stone of the appropriate tier. Repairing equipment fully restores its durability rating but decreases its maximum durability potential by five, to no less than one. Maximum durability is restored if the equipment is either enhanced or enchanted successfully.