Most gear can be found in a useable condition, but wear, tear, and the passage of time can begin to reduce the quality of your equipment. Getting hit particularly hard, or being bathed in acid can be the cause of a reduction in durability, for example. When an items durability is reduced, the is a specific downside, depending on the type of item. When an item’s durability reaches 0, it breaks. Repairing a broken item is very difficult, and can usually on be done by masters of their craft, or magical means.
Weapons have 5 maximum Durability. Until the weapon has reached 0/5 durability and has broken, losing durability has no effect. When a weapon is broken, it can not deal damage from its damage dice - only from it’s bonuses, and from other sources, such as an enchantment of flame.
Higher quality weapons also gain a bonus of durability equal to their quality level. When a Higher Quality weapon loses durability, it loses its higher quality bonuses alongside its extra durability, until it reaches 5 durability, thus having no additional bonuses from its quality. Repairing the weapon, causing it to regain its bonus durability, causes the higher quality weapon to regain its bonuses from its quality level.
Armor has durability equal to the amount of base armor granted by the armor. So, Plate Armor has 20 base AC, and also has 20 durability. Higher quality equipment also has their durability increased by their quality level. So, +1 Plate Armor would have 21 durability.
When Armor is below its maximum durability, it loses base AC by the amount of missing durability. So, 18/20 durability Plate Armor would have a base AC of 18.