This article has a lot of info on the subject. In summary: Cubemaps are basically 360° screenshots (think Google Maps street view) of the map that reflective surfaces use to show what is around them. You make them by placing env_cubemaps in your map and building cubemaps via console (a step-by-step process is outlined in the VDC article).
They are actually relatively simple to create in hammer, but actually building them in-game so they work is a bit more of a process. It's still easily doable though if you look at a tutorial.
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