Condition

Condition, commonly known as durability, is a weapon, tool or armor's state of repair. As an item's condition worsens, it becomes less effective, either dealing less damage, providing less armor points, or less healing/armor repair. When an item's condition reaches 0, it becomes useless until it is repaired.

Most vendors (weapon, armor, medical, crafting) can repair your items for you, through their repair tab.