Digital cameras use something called "contrast detection" when they focus on a subject. I'm not going to go into the technical explanation of how it works since it doesn't really help with this question.
When light levels are low, the camera may not be able to decide on a focus distance. Some cameras won't take a picture, while others default to some preset focus distance (usually infinity). This can be a real problem.
That's where AF illuminators -- also called AF assist lamps -- come into play. They cast a bright, focused beam of light -- usually white or orange -- onto the subject, so the camera can "see" a little better and focus properly. Other cameras use infrared or laser-based focusing, but it's the same result in the end.
What is low light exactly" For me, that's indoors without very strong lighting. There are some cameras that I can't even do the redeye test on because they won't focus.
Cameras with an AF illuminator do a much better job in these situations, and that's probably why you'll want one. If you do lots of outdoor or studio photography it's not a big deal,but for indoor candids, it's a good idea.