Introducing the Lightmap (Page 1 of 3) by satchmo · 45,977 views
This topic explains how lighting interacts with the textured surfaces in the map. A lightmap controls the density of the shadows. In another word, it's a way the mapper adjusts how "sharp" the shadows appears on the surface of the architecture.
If you're a careful observer in real-life, you'll notice that the edges of the shadow from objects that are far away from the ground are blurred. Whereas the edges of the shadow cast by something relatively close to the ground appears sharp. We're not going to go into quantum mechanics and the properties of photons, but pay attention to the shadows around you, and you should be able to see what I am talking about.
The Source engine is superb enough to take this property of photon into consideration. It allows the mapper to control the sharpness of the shadow, or as true mappers call it, the density of the lightmap.
Adjusting the Lightmap