The durability rating of an item is a measurement of how well that item withstands structural damage. The durability rating of a weapon or wearable item is usually based on its material.
Whenever an item is misused or might be damaged, such as when a melee weapon misses a swing and hits a solid stone wall, the GM will call for a Durability Check, which is a 1d20 natural roll. If the outcome of the check is higher than the item's durability rating, the item's condition deteriorates by one level.
As such, an item with a durability rating of zero is guaranteed to deteriorate any time it is misused or damaged, while a durability rating of 20 makes an item effectively indestructible.