The concept of Yin and Yang permeates all aspects of Chinese Medicine. Some people tend to think that its simply the notion of opposites - light and dark, day and night, masculine and feminine, etc.
There is some truth to this, but the concept of Yin and Yang is way more dynamic than that. There is a wholeness and movement that can be seen in the Yin/Yang symbol (Taiji Symbol).
Yin becomes Yang and Yang becomes Yin. And there is a little of each within the other.
Here is a great video that beautifully (and simultaneously) shows the dynamic nature of Yin and Yang.