Durability is the measure of a number of hits that a weapon can make before it takes a penalty, or "breaks". A weapon can be repaired at any Blacksmith, or via the Staff of Forging, or if not completely broken, can be slightly mended by the Maintenance talent. Missing with a weapon will not decrease Durability. Guns lose one Durability per attack, not per shot.
Durability can be affect by enchantments, ores, weapon parts, or upgraded at a Blacksmith.