"Y" is for Yin-Yang
The yin-yang symbol represents the ancient Chinese Taoist idea that there are two energies - the yin energy and the yang energy - that created the universe, and oppose each other to keep the universe in balance. If you go too far along one, it becomes the other. A great example of this is temperature. If you put your hand in a bucket of ice water at first it feels really cold. But if you leave it in there longer, you will feel a burning sensation - just like heat.
Yin-yang is the unifying principle in Traditional Chinese Medicine (including proper diet and exercise).
You can find a very interesting pictorial explanation of how this symbol was developed here.
And here's what wikipedia has to say on the subject.
There's much symbolism within the symbol itself, and there's a myriad of interpretations. Here's a few of the ones I like: