If you can imagine the polygons it takes to make up an object, such as an airplane, being replaced with a camera and what the camera "sees" being mapped to the faces of a cube? That's a cube map, or reflection map, or environment map. How often the camera "snaps" an image to update the cube image is what the slider does. It's not a perfect solution, but at least you can have 'some' reflections created by the environment around you. It may be two or three generation more before GPUs can do it in real time, but by then we will be thinking about sub-surface scattering and even smaller details I think.