Recirculation is the process of pulling the wort from the base of the mash tun or lauter tun and recirculating it back on to the top of the grain bed. Recirculation typically occurs after the end of the mashing process. As the hot wort is recirculated through the grain bed of the mash, the grains act as a particle filter clearing the wort. As the wort is recirculating, it becomes cleaner and less turbid until finally it is clear and ready to be passed to the boiling vessel.
A pump is typically used to recirculate the wort at a steady and controlled pace. In the case where a home brewer does not have a pump available, the wort may be drawn into a container and slowly poured back on top of the grain bed. The process can be repeated until the wort has become clear. Additionally, rice hulls may be added to a mash as a means of boosting the filtration capability of the grain bed.
A wide selection of home brewing recirculation pumps can be found here:
Wort recirculating in the mash tun, prior to sparging and transferring the wort to the boil kettle.