This section will probably make sense only for people who export models to use them for advanced purposes, and not so much for people who run everything within SketchUp
Illum surfaces are useful for rendering and modeling luminaires. In fact, if you play with Luminaires in groundhog, the surface that actually emits light is an Illum.
Greg Ward once helped me define illum surfaces as:
Illum surfaces, also known as "impostors" or "proxies," are light-emitting surfaces whose luminous distributions are equivalent to the distribution transmitted through them as the consequence of the existence of other (direct and indirect) light sources. Mkillum is the program that allows creating illum surfaces in a convenient way (Ward & Shakespeare, 1998). For example, the daylight distribution transmitted through a window, which can be considered a consequence of the existence of the sun and sky, can be summarized into an illum surface.
Illum surfaces are created (Labeled) in the exact same way as Windows. That is, right-click / Label as Illum. Please read the "Adding Windows" tutorial for more details.