When your equipment's Condition wears down from use, it becomes less effective. To restore your equipment's condition and effectiveness, you will need to repair it. There are two ways of doing this:
NPCs available in every town in the game can repair your equipped items for gil, with the price based on the strength of the item and its Condition. However, they can only repair items to 100% Condition. This option is fairly easy and does not require any special items.
To repair items fully, you must use a Disciple of the Hand class that is no more than 10 levels lower than the item you wish to repair (indicated on the tooltip of the item.) You will also need differing levels of Dark Matter, depending upon the item level of the piece(s) you wish to repair. The class needed is dependent on the type of item as well. You can determine the needed craft by examining the item in your inventory.
Repairing with a Disciple of the Hand class adds 100% to the condition of the item, up to a maximum of 199%. You cannot use repair on an item that is 100% or higher, it must be 99% durability or lower to repair.