Here are some excellent explanations given to me in the Above Top Secret Forum! Wow this guy really has a grasp on it!http://www.abovetopsecret.com/forum/thread592526/pg1
I think the premise is that...
1) The reality that we observe is not necessarily an accurate portrayal of what really is.
2) What we perceive of as planets and stars are merely our perceptions of...something. What exactly they are doesn't matter at this point.
3) Those things radiate. What do they radiate? "Vibrations." What's vibrating? Don't know. Doesn't matter. But whatever it is, just like how ripples in a pond will affect the course of a leaf floating on the water, so too will the vibration from these plantary sources influence the path our lives take.
4) Different planets radiate at different frequencies. And just like different frequencies of light manifest a distcintly different character (Even though "red" is actually quite a broad range of individual frequencies, it manifests a character that is clearly distinct from yellow, right?) so too do planetary vibrations exhibit their own distinctive "character" in the manner in which they influence the course of a life. The "Jupiter" influence has a character that is distinct from the "Saturn" influence.
5) The "sun" that we perceive as a ball of gas in our solar system, like the planets, is similarly just our preception of a "something." In the case of the sun, that something is, generally speaking, a manifestation of our local connection with the masculine force of divinity. The "doing" principal of energy. The angle of alignment of any particular planetary influence with that force determines to some degree the "strength" with which that influence pass through us. Sort of like how the sun is "strongest" at high noon, because its rays hit us at a 90 degree angle, and so less is deflected.
6) When various alignments between occur between these various objects, interference patterns may result in their vibrations. These interference patterns may themselves influence the path of our life just like the direct vibrations themselves do.
7) At the time of our birth we are either "imprinted" with the distinct character of the sum of all vibrations available at that moment, or we as spirit-beings choose to incarnate at a time in which those vibrations are condusive to the sort of experience we seek to have. Thus, time of birth determines what sort of person we are likely to become.
8) Similarly, at any given time, the sum total of those vibrations determine what sort of experience is available to the societies, the cultures, the world, etc. The "age of aquarius" has a tendancy to exhibit a different character than any other, because those particular influences are most dominant during that time.
9) Ultimately, however, the relationship between these vibrations and the course of lives and societies exposed to them is not one of direct cause and effect, but rather, one of what experience is available. Human experience behaves like an antenna. An antenna will only receive the sort of vibrations that it is tuned to receive. Consequently, while one may have been born under the influence of a particular planetary vibration, one will only manifest its character in ones life if one is suitably tuned to receive it. Times, planets and societies will similarly only manifest events according to their ability to receive them.
At least, that's the theory as I understand it. I've never really followed astrology much.