The toolbelt is an accessory that adds five additional accessory slots. It does not take up an accessory slot itself. Like other accessories, it cannot be stacked. This means if you buy 2 toolbelts, you will still only get 5 additional slots. Buying multiple toolbelts will not give you 5 more slots each time.
How to Obtain
The only way to obtain this item is to buy it from the in-game shop. It cannot be traded.
How to Use
The very act of purchasing this accessory "equips" it. You don't need to keep it in your Accessory Bar.
Toolbelts were added in September 2013 as part of the 1.12.0 release.