The concept of Yin and Yang is fundamental to a true understanding of Wing Chun, or most any form of Kung Fu. The concepts behind Yin and Yang developed from simply observing the natural world.
The very act of seeing or observing divides the world into mutually dependent opposites, each giving meaning to the other. For example, “light” has no meaning without “dark;” “up” is meaningless without “down;” and “inside” does not exist without an “outside.” Yin and Yang are not “things” in the Western sense of the word, but rather are descriptors that show relationship or dynamic interaction with an opposite.
The Chinese character for Yin translates to mean the “dark side of the mountain,” and represents such qualities as darkness, stillness, cold, passiveness, softness, quiet and potential. The character for Yang means the “bright side of the mountain,” and represents light, action, heat, aggression, firmness, loud and expression.
The Taiji Jee or Yin-Yang Symbol also illustrates a key aspect about the interaction of these two forces. They are not mutually exclusive, but rather are constantly interacting and even transforming from one to another. The dark spot inside the white field represents the potential for Yang within Yin, and the blending shape illustrates that there is a transformational balance between these two qualities.
Learn more at Wikipedia.