uberEnironment is a light, but unlike the spot or point lights it doesn't come from a specific place, and unlike the distant light it doesn't come from a specific direction. It produces light from all around the scene, either a flat light or a light that varies by direction according to an image map (so it can fake the way a real object is lit by the light bouncing off or scattered through objects in the world around it) - in fact you have the option to do true global illumination which actually works out how the light bounces from the real items in the scene, rather than faking it with an image. Ambient Occlusion is another trick - it darkens areas that are near to other things, either in an indentation in a single item or one item close to another, to imitate the way recesses get less light than exposed areas.