Yin and Yang are negative and positive principles of the universe. One cannot exist without the other, and they often represent opposites in relations to each other. As you have more and more Yang, eventually, Yin will appear and replace this increase. Similarly in the opposite direction, Yang will appear to replace the increase in Yin.
The Yin Yang symbol (circle with black and white sections) depicts this clearly. As you travel around the circle, white or black will increase, until the opposite color is almost gone, but never totally gone. The cycle then repeats for the opposite color.
What seems like Yin is often supported by Yang, and vice-versa. As an example, to truly know good, you must know what evil is, and without good as a comparison, nothing is evil. Thus, while keeping to one end, do not shun the opposite end, but embrace both as they are. Allowing Yin to flourish, you welcome Yang. By letting go of Yin, you are waiting for its return. As an example, before you can possess something, you must be willing to let it go. Yin and Yang