Every Weapons and Armors have durability that decreases every time the item is used.
Once the durability reaches 0, the item no long functions properly and needs to be repaired.
Items' durability can be upgraded through refining procedure.

[Refine Enginner]Astin in Lonbarei Training camp in Horusland, [Refine Engineer]Hestia in Magnaksia Military District in Kether, located in DeFugel are in charge of Refining.
In Croxion, [Refine Instructor]Kalbert in Kuznets Advanced Base in Yesod and [Refine Instructor]Latmo in the Headquarter of Blo Brunak Magic Knights in Malkuth.

Double click with left button on the mouse or right click on the NPC, then Item Refine window appears. Place in item that needs to be refined in the slot where it says refine item, and place catalysts into the slots to refine. Maximum of 3 catalysts can be used at once.
Click on Refine button to start the procedure.

If Refine procedure succeeds, the item's durability and refine count increases.
The items used as catalysts will disappear whether the procedure succeeds or fails.
However, even the procedure fails, you will not lose the item that tried to refine.