Hi bckiceman!
Don't let the terms yin and yang mislead you into believing they are some kind of new-age mumbo jumbo. They are Eastern and not familiar to many Western ears, but they are actually a very rational and scientific way of describing and delegating substances/concepts. They are also central, so very important, to the macro way of life.
What books are you using for your macrobiotic practice? I recommend The Self-Healing Cookbook by Kristina Turner, and Aveline Kushi's Complete Guide to Macrobiotic Cooking. They both give really practical, accessible explanations of yin and yang. ON EDIT -- FOOD AND HEALING by Annemarie Colbin is definitely one of the best. How could I have forgotten? *smacks forehead*
In the meantime, check out this
article from CNN. It gives a good, short explanation of macrobiotics and the principles of yin and yang. There may be some good info in the articles on Cybermacro too, but I think this site is lacking a really good "newbie basics" page.
Understanding these principles of deficiency and excess/expansion and contraction and how they affect your body are key to being able to cure illness, and understand the world a little more clearly. I think others may do more justice to the theory than myself, first thing in the morning, so I won't attempt a full explanation right now. But I urge you to learn -- I promise, you'll be happy that you did.
mara jean