i THINK its like spacebuild maps im not sure

https://www.youtube.com/watch?v=haEnMYAL-ho Watch this... You will understand it after this...

Cubemaps are a technique to simulate reflections. Every reflection you will see in the Source engine is done with cubemaps.

When a map is built, after compiling the map so it can be played in game. The cubemaps need to be generated in game as well. Basically you tell the game to sample your map on a number of selected locations to generated a set of images that will form a cubemap that serves as the images you will see in the reflections of other objects.

Every map should have these to properly show reflections. Of course not all maps have the same quality and some maps do not have cubemaps and as a result they cannot show reflections properly.