This guide will let you know what Mending does in Minecraft.
Mending is an enchantment in Minecraft that uses XP points gained to repair items automatically.
The durability of an item decreases the more you use it.
When you do activities that generate XP, Mending will use that XP to repair the tool you are using.
It will make the durability of an item a lot more than just using unbreaking III. You must be holding the tool for mending to take effect.
If you don’t hold it, it will collect the XP into your XP bar instead of mending the item.