The jig is made of plywood and sized to fit your router table. A piece of aluminum has a series of holes sized to fit a pin. The pin becomes a pivot point for your workpiece. A pair of cleats on the ends of the base keep it from sliding side-to-side.
To use the jig, position it so the edge of the bit to the center of the pivot pin is the desired radius of your circle, then lock your router fence in place. This acts like a stop. Then, slide the base away from the bit and fit your workpiece over the pin. After turning your router on, you can push the base to the fence, engaging the bit in the workpiece. Then, simply spin your workpiece to create the circle.