It seems a lot of players think the first Anvil you’re given via a quest is used to repair your equipment, but the Anvil is used to improve your equipment. To repair, you must travel to the main city of Klaipeda to speak with Zaras the Blacksmith.
Zaras can be found on the south side of town, near the Item Merchant.
Zaras allows you to repair, buy equipment recipes, and add gem slots to your gear.
Equipment durability can be whittled down pretty quickly when you’re mass-grinding mobs in Tree of Savior. The game will let you know when your durability is critical or at zero. Keep an eye out when you’re grinding and repair when you need to.