German for "recirculation."

During the vorlauf process, wort is drained by gravity into a vessel known as a grant. The wort is then recirculated by returning it to the top of the grain bed in the mash tun.

The purpose of the vorlauf is to remove the particles of grain that remain under the false bottom in the mash tun. If the particles were transferred into the boil, they could cause unpleasant flavors in the finished beer. 

Brewers continue to vorlauf until the recirculating wort runs clear.