VIS stands for Visability Matrix. It processes all of the rendering and light bounces. It basicly controls what is rendered and how. If you compile somthing without VIS it goes fullbright just like Zl said. The reason that happens is because RAD (the lighting program) relies on VIS to calculate the bounces and renders! So if VIS doesn't run, RAD can't run either, causing it to be fullbright!
In big, complex multiplayer maps, you should put in vis blocks. This means that only certain parts are rendered at once. You have to be very careful where you put these otherwise your map will render very funny (because the vis block will render as nothing making a block of black!). Vis blocks are good for keeping r_speeds down in maps. R_speeds make everything run laggy and chuggy! This is espescially important in multiplayer maps!
Did that explain it? I spent 8 minutes working all of that out!