Originally the terms referred to the two sides of a hill; one in the sunshine (Yang) and one in the shade (Yin). Many of the phenomena of nature display these two aspects, most obviously in the way in which daytime (Yang) passes into night (Yin) and then back to day again. Thus this balance is a dynamic, moving one, in which Yin gives way to Yang which in turn gives way to Yin again. Yin depends on Yang, Yang depends on Yin.
Yang is thus to do with light, warmth, the sun, fire and activity - we are active in daytime. Yin conversely is to do with darkness, cool, the moon, water, stillness. In terms of the human being, balance most obviously comes down to a balance between waking and sleeping, between activity and stillness, between warmth and coolness. However, these simple concepts grew into a sophisticated understanding of the natural world, and of the human being as a microcosm of that world. In terms of medicine, someone who is red faced with a loud voice, who is thirsty and drinks in large gulps, who is restless and perhaps can’t sleep well, and who complains of thumping migraine headaches is displaying a lot of signs of having a bit too much Yang going on. This may in part be constitutional, but thumping migraines aren’t part of anyone’s constitution and treatment for this person should aim to ‘subdue Yang’. On the other hand someone else may be tired and listless, always inclined to feel the cold, pale and quiet and prone to dull, heavy headaches especially when the weather is cold and wet. This person is too far over on the Yin side.
To take the analysis one step further, we can start to ask whether the person with apparently too much Yang is not in fact really just lacking in Yin. For example, someone like the red faced person above is to be distinguished from someone who just gets hot at night, or who has hot flushes, whose face is not red all over but only on the cheeks ( a malar flush) and who is thirsty but only drinks in little sips. This person is lacking in Yin, and treatment for them needs to nourish their Yin rather than to subdue their Yang. Thus there are four basic treatment strategies; to nourish Yin, to nourish Yang, to eliminate excess Yin and to eliminate excess Yang.
However, human beings are far more complex than that. If someone is, say, Yin deficient, we need to know which organ system in particular is affected. If in addition to getting hot at night and displaying a malar flush, they have a dry, irritable cough and a hoarse voice, it sounds like it is the Yin of the Lungs which is depleted. If on the other hand they have a dull ache in the low back, feel a little dizzy and their urine is dark and scanty, it is more likely the Yin of the Kidneys.
And then again, someone could have Yin deficiency of the Lungs, but Yang deficiency somewhere else; the pattern will probably be quite complex, especially in the case of someone with a chronic illness. It can get more complex because Yin and Yang form a dynamic pair so that Yin transforms into Yang and vice versa.
These concepts are ancient, but they still provide a powerful way of understanding the human condition and human illness. Going back to the red faced person with excessive Yang and migraines above, it is more than likely that if this person can learn to bring more Yin into their life - something like meditation, relaxation, some kind of inner work - their headaches will start to subside. They might need some help from medicine as well, whether Traditional Chinese or modern western, but if they can understand that their migraines are a message telling them that their life is out of balance, they can do a lot for themselves.