Durability

In early levels, durability is hardly an issue, people change armour as they level up too frequently for it to matter but at higher levels, especially when the armour/weapon is well upgraded and enchanted, it will be used for a very long time.
Items, with the exception of rings/talismans/necklaces, will lose durability from either deaths not including kingdom war, or from taking damage in the case of armour, or hitting mobs in the case of a weapon.
Durability is not a big an issue as people make it out to be, you can play the game forever and never need to buy a spirit forging hammer from the item mall.
When durability reaches 5 or lower, you should head to a npc, such as a pawnshop or any selling npc will do and select repair and click on your item with the little hammer cursor.
When you do, your 5/100 item will become 90/90 and in future 80/80, 70/70 down to the minimum of 10/10 after which it will drop no further than 10/10 after each repair.
The spirit forging hammers in the item mall come into play when you want to remove these durability penalties on items, just use it on your weapon/armour and 10/10 will become 100/100 again.
You might wonder why anyone would need more than 10/10 durability if they can just repair often and save money but there is one specific case that causes durability issues.

Its when you use zhang zhao or xun yu martial spirits to boss mob grind for fast leveling.
Since durability on a weapon depends on how many times you hit a mob to make it go down, then it goes down very fast when using their damage reflection skills, 20 hit you continously at once till they die, then they respawn again, repeat for hours and hours, weapon loses durability extra fast.
There are times i've had to repair weapon twice a day, that was during a double exp weekend when i was rushing to level 150.

As a note of warning i should mention that in the past a GM gave wrong information about durability.
Mean had a +20 wolf fang hammer, he was told that when durability reaches 0/100, it would just become 90/90 by itself and that it would be ok.
When it reached 0/100 the axe actually broke, it became a destroyed item that required an intermediate repair stone to restore.
This may have been a bug and it may have been patched and fixed now, but this was a problem in the past, so just in case, please repair before it reaches 0/100