Also known as the force of all wearable items in Nexus, Durability is an item's % of remaining consistency. Remember that all items that can be worn have a durability.
What is Durability's Purpose? Well, Durability, or Dura, is all that remains of a weapon, armor, hand item, or shield. If an item's durability runs out, then it will fall apart into dust. The feature "repair" allows you to bring your Durability back to full health on most items. Some items can not be repaired via an NPC, and these must be repaired by a Sam San Warrior.
You are able to see your item's Durability by right clicking it in the menu and looking at the top numbers. The ratio will look like XX/XX. For example 20/100 is a durability, therefore, only 20 points, or 20%, of the item remains.
Dying can decrease the durability by an item drastically. Be sure to keep your items repaired, or risk losing them.