Magnets affect materials in one of three ways, attraction, non-attraction (neutral) and replusion. In a homopolar motor the flow of electrons through a wire creates a magnetic field that is in oppostion to that of the disc magnet and thus is repulsed by it. This repulsion creates a tangential force called the Lorentz force that is reponsible for creating the spinning motion. Dependent on which item is fixed the opposite item will spin. In this case the copper tracks are fixed so the magnet rolls along it
This simple Roller is composed of simply a battery 2 magnets, two copper tracks and a piece of wire.