An eddy current is an electrical current induced in a piece of metal due to the relative motion of a nearby magnet. Any time a magnet passes a metallic object, its magnetic field induces an electric current, which swirls around near the surface of the metal like an eddy in a river. This electric current creates its own magnetic field, which opposes the motion of the magnet.
Eddy currents are responsible for some remarkable effects when properly utilized. The opposing magnetic fields produced by eddy currents can result in marked slowing of the motion of a magnet, even when it is otherwise in free fall. A strong magnet dropped through a thick copper tube, for instance, falls far more slowly than it would under the influence of gravity alone.
This effect is only observed when magnets move near metals, not nonmetals. This is because of the nature of the metallic bonds in solid metals, which allow for the relatively free flow of electrons between atoms. This is the same property that is responsible for the generally good electrical conductivity of metals. Because of this property, it is relatively easy for magnetic fields to start an electric current, which is just another way of saying electron movement.