The original meaning of yin was the shady side of a hill (the development and transition of the character depicted above, from original to the modern, left to right). It has become widely known as the counterpart to yang, representing the feminine aspect and the innate potential for all actions and events.
Yin is the innate complement to the active and evident. Where there is movement, there was stillness prior, and there will be subsequent stillness. Where there is fire, there is fuel.
Yin and yang are descriptions, denoting the dual nature of an entity or event. They are interdependent, and forever flowing, blending into one another.
They are relative terms. Something can only be 'yin' in comparison to something that demonstrates a yang-type nature. For example, in Chinese medical anatomy, the lower parts of the body are considered yin, but of the knees and feet, the feet are yin.
They transform onto one another; watch any event, and you will the the rise of action, lead to stillness. And they exist, as seeds, within each other; it is from within stillness, movement is born - rest finds its birth, its origin, its beginning within activity...