Don't really know a lot about it, but the base concept seems to be that the universe is divided into two concepts--Yin, or the female, and Yang, or the male. Everything in creation has either one or the other as a...'vibration', or internal set of traits and spiritual power.

They're also complimentary; you can't have one without the other, even though most things supposedly lean one way or the other of their own accord. too much of one over the other is considered an imbalance.

Other than that, I really don't get the whole concept. And I probably have half of that wrong, anyway. I wasn't raised in that system.