To vorlauf is the action of running off some wort from your mash/lauter tun into a container, then putting it back on top of the grain bed.

To vorlauf is the action of running off some wort from your mash/lauter tun into a container (like a bag measuring cup or small bucket), then putting it back on top of the grain bed. This allows the grain bed to be used as a filter and eventually the wort will start to come out clear. At that point the vorlauf is completed.