The "lower" planes and references to "lower" spirits refer to those spirits that tend to possess an energy that quite simply feels lower. There's really nothing to do with the illuminati or any other such conspiracy theories that is related to them, though I'm sure if you tried you could dig up something.
For the most part, low spirits are simple consciousnesses that do not much possess the the capability to feel or comprehend more complex emotions. As such, some of them are feared because they attempt to manipulate and experience the more base emotions such as fear, anger, jealousy, etc. so as to know something closer to the depth of love, happiness, and so forth.
Most are either spirits tormented by madness, spirits that have been "broken" in some way or another, or spirits that have otherwise developed without this complexity.
The lower astral, or the lower planes, typically refer to those areas where such entities exist.