It is a device that employs a rotating magnetic field to cause a stir bar (also called “flea”) immersed in a liquid to spin very quickly, thus stirring it. The rotating field may be created either by a rotating magnet or a set of stationary electromagnets, placed beneath the vessel with the liquid (source: Wikipedia)

This is very common equipment in a bio lab. We have developed our own DYI-magnetic stirrer. Here you can see how it works with a portable battery.