This is a simple holder for snap-off style utility knife blades. It can be used to make consistent cuts in soft materials such as foam sheets, cardboard, paper, fabric, leather, etc.
To use it, simply insert the tip of a knife blade into the desired slot (slots are spaced on 1/4" cutting intervals) until it sticks out the bottom face. Angle the blade so the front and rear are supported by the holder. Then slide the cutter along the edge of your material or cutting mat/table to make the cut.
For thick materials, especially foam, it's best to make multiple passes, starting with a very shallow cut to form a guide groove, then extending the blade more each pass.
The blades are a fricton-fit install and are easy to remove, but don't forget to be safe! It's best to use pliers to remove the blade.