An illumination is simply a colorful illustration found in a book. However, it usually has an outdated look, because many times these illuminations were done some 600 years ago, and tended to accompany the pages of text written and scrolled on by monks, who were often the only people who could read and write in those days.
These illuminations never fill the entire page, but are simply inserted into the beginning of the chapter, up on the top left.
The illuminations often are very poorly drawn, though they are colorful and have nicely done borders (there's that word again) around them. It is painfully clear that those who rendered these illistrations were by no means artists, and were simply instructed to draw something out, according to the contents of the chapter.
I think the illuminations merely have an historical value, if sometimes grotesque and not truly "cultural" in quality, but they are worth noting for the sake of historical (hysterical?) drama.