This is a safety beveller, which is used for thinning and chamfering down your leather. For example, if you are making yourself a belt, you may need to thin the leather to fold it neatly around the buckle. You may also want to chamfer the end of the strip, so that there is no uncomfortable 'step' of thick leather on the inside of your belt.
To use the beveller, rest your leather on a cutting board or bench, rest your beveller on the leather and using light pressure, push the blade away from you to remove the excess thickness from your leather. If you haven't used a beveller before, it's best to practice on a spare scrap of leather before you use it on your craft project. Be sure that you always keep your hands behind the blade, as it is very sharp!
This tool has a removable blade which you can slide out and replace when dull.
One blade is included with the tool, and replacement blades are available below in related products.