Yin and yang as a philosophical concept to me (and I think the dictionary) represents the complementary and interconnected nature of apparently opposed forces, most significantly of "good" and "evil". It is a way of illustrating that those forces (ideas) are both linked and inescapable. One cannot exist without the other.
Yin and Yang are opposite qualities that represent much more than good and evil.
It's a philosophical standard that you may not have one thing without the other. The universe is always in balance and to do away with evil would be to also remove good. They also represent qualities of femininity and masculinity.
Most notably I think are the small spots within them. Despite being opposite, they carry qualities of each other within them. Yin may be the opposite of Yang, but Yang is still within Yin - and vice versa.
Its a very beautiful concept, but it can be quite frustrating ~
To me it's a lot of things. The balance of masculine and feminine, dark and light, good and evil, and shows that all has a touch of its opposite within.
As said before, you can't have one half of a spectrum without the other, and there is no force that's 100% pure in this world.