What is the meaning to Yin and Yang?

Yin and yang are part if the taiji, and refer to the intrinsic unity of opposites within a greater whole. Symbolically there is an empty circle (called wuji), and some kind of movement in that emptiness gives the appearance of separate elements - one rising (yang), one falling (yin) - which as a whole are bound together (the yin elements filling the voids that the yang elements leave behind). The traditional metaphor is sunlight on a mountain: as the sun moves across the sky, some things are brought into light and other drop into shadow, giving an impression of motion where in fact everything is perfectly still. A lot of natural dichotomies are cast as yin and yang (female and male, cold and hot, moist and dry, diffuse and concentrated, low and high) in order to highlight that they are in fact unified at a more basic level.