The theory of yin and yang is an important part of Chinese Medicine. It is very old, first mentioned in the historical record in 700 BC. It is based on observation of nature in which cycles alternate with one blending into the other and no clear distinction in between. In nature there is typically a gradual shift and flow, not abrupt and total change, with a balance from one end of the continuum to another. Because of this things that seem opposite can actually be complimentary and relational, with qualities of one thing only being defined by comparison to qualities of another thing. Examples of this would be the shift of day into night, or the gradual change of seasons.
The same holds true for the human body. We shift gradually over time in small increments, from children to seniors. We go through daily cycles of activity and rest, and seasonal shifts as we adjust to changes in our environment. Within our bodies there is also a balance of opposites: substance, which is yin, and function, which is yang. Yin and yang depend on each other and must be in balance with each other in order for the body to work properly. Yang processes, transports, and excretes, while yin nourishes, enriches, and moistens.
Many illnesses are rooted in an imbalance between yin and yang. There is a special category of herbs in Chinese Medicine called tonics which can help replenish yin and yang, addressing these conditions at the root and preventing them from reoccurring. Yin tonics are herbs that generate fluids, moisten dryness, and prevent the body from overheating. These herbs treat a variety of common disorders such as dry cough, excess thirst, dry skin, and dry constipation. Specific examples of yin tonics include American ginseng, Solomon's seal, lily bulb, and black sesame seed. Yang tonsics are herbs that warm the body, increase metabolism, stimulate libido, and improve the functioning of the internal organs. They treat coldness, fatigue, impotence, diarrhea, and frequent urination. Yang tonics that you may know include walnut, black cardamom, fenugreek, and astragalus seed.