An entity that takes pictures of the area around it. These pictures are used to help reflective things (like glass and water) near the cubemap reflect properly, among other things. Any time you make glass or water, you should place an env_cubemap entity near the reflective face and use its Brush Faces property to choose that reflective face. If you are not trying to make a face reflective, you can just place the entity and forget about it.
The VERC has a cubemap explanation that basically amounts to: "use an env_cubemap near reflective surfaces or in areas with a high visual contrast, such as a bright yellow room next to a dark blue room."
You must type buildcubemaps in the game's console then reload the map to get cubemaps a-working.