Please read the tutorial first before asking questions. Carefully study this tutorial:
http://cariad.co.za/twhl/tutorial.php?id=35This is an excerpt from the tutorial:
Although this program doesn't have to be run to make your map work, and you may not even notice a difference when you do run it, it is crucial for final compiles. VIS calculates which polygons are visible from every point in the map, so that only polygons visible to the player are drawn at each point. If not run, every polygon will be visible from every point in the map, which can cause major performance loss in big maps, or on slow PCs.
But you still have to read through the whole thing to have it make sense.
This rule always apply: read the tutorial first before posting questions on the forum. I'll let it slide this time, but another member might not be so nice to you.