Durability is a property of weapons and armor; it is a measure of a piece of equipment's physical condition. All equipable items (besides rings or ammunition) have a fixed number indicating how much damage can be sustained before the equipment breaks. Receiving damage in most forms (except from Poison Cloud, Death Cloud, poison water, plague water, lava, touching a Fire Lizard, or falling damage) will reduce the durability of any equipped armor, while blocking incoming damage or attacking with a shield or weapon will reduce the durability of the weapon/shield used. The Acid Cloud spell and the Scraping Spear damage all of the player character's equipment, at a significantly faster rate than other damage sources.
Once the current durability of a piece of equipment is at 30% or lower of the maximum durability, the item will "break" and will have greatly reduced defensive properties (in the case of armor) or have greatly reduced attack damage and guard break reduction (in the case of a weapon or shield). Blacksmith Ed or Boldwin can repair equipment in exchange for a number of souls relative to how much damage the equipment has sustained and the rarity of the item. Alternatively, an Ed's Grindstone can be used to restore some of the durability of the weapon in the right hand. The Ronin's Ring reduces the rate at which weapons degrade.