Repair Tools can be used to repair armor and weapons. To use them, equip them like you would a weapon, and a window will appear showing a list of all damaged armor and weapons. You can click on one of the items in an attempt to repair it. Your chance of success and the amount of damage repaired depends on your Armorer skill, and the type and quality of the tool you're using. The tools have a limited number of uses before they break, so you should carry a decent supply of them if your skill is low and you can bear the weight. There are five standard grades of Armorer's Hammers and only one grade of Repair Prongs. All these are available at vendors.