Each character in your party is able to equip two accessories. Each accessory that you equip can range from items that help your defense, like armor, to items that raise stats. Some accessories even allow you to learn new spells or to use ones already acquired.
Accessories can be found by numerous different circumstances throughout your travels in Spira. For one, you can buy accessories in shops. Many shops sell the basics, then continue to sell items that get better and better as time progresses. You can also gain accessories by stealing items from enemies or bosses, or just by killing bosses.