Vorlauf is the process of clarifying the wort being drawn out of the mash tun. Often this is as simple as drawing 1-2L of wort at a time slowly into a container and then pouring softly back onto the top of the mash; taking care to disturb the grain bed as little as possible.
The primary objective is to establish the grain bed as a filter for the wort being drawn off. The grain husks will then act as a sand-style filter to further enhance clarity.
When the wort runs mostly clear (meaning no large pieces of grain material) the vorlauf process is complete and sparging may begin.