VVIS or Valve Visible Information Set is the command-line tool that takes a compiled BSP map and embeds visibility data into it. VVIS tests which visleaves can see each other, and which cannot.
Without "visibility", everything in a map is drawn all the time regardless of whether or not the player can see it. Clearly the list of objects to be rendered must be trimmed, but how? It would take far longer to test whether each object or surface could actually be seen than it would to render them in the first place!
Game engines have to make a compromise; Source's is the Binary Space Partition model, based on John Carmacks implementation in Quake.
Each visleaf (sometimes referred to simply as a leaf) is a hollow volume in a map which defines a 'cluster' of visible surfaces. Every visible surface of a map is part of one visleaf or another. Visleafs are used primarily by the Rendering Engine to determine (before rendering each frame) which areas of the map might need to be rendered on screen. When any part of a visleaf is potentially visible from any part of the current visleaf, the entire contents of that visleaf are considered for rendering.
Ill make a guide when i'm done with my map explaining stuff like this, but in english.VVIS.exe is the second process in a compile. It takes the different visleafs that VBSP.exe makes and calculates which can see which. This information is mostly used by the ingame renderer, but many other things, such as VRAD.exe, depend on it as well.
All compile process except for VBSP.exe give you the option to disable them or just run them on "fast", which is usually as bad as not running it at all.
Nick is making the assumptions that you might not be running it, and that soundscapes use visleafs.