Cubemaps are series of 'images' taken from within the game that are used for reflexivity purposes. By placing an env_cubemap in a map, you're giving the engine a marker for a reflective area.
When you build the cubemaps, the engine takes 'images' of what's in the cubemap's immediate areas and saves them. These images are called up when textures request them.
Say you have a tile texture. The tile texture would have a property in it that asks for the cubemaps. This technique is used to produce a reflective or shiny surface, as it essentially takes the cubemap image and maps it to the texture that asks for it.