Yin Yang is a philosophical concept of balance. The symbol associated with this concept is described by Elizabeth Reninger in her article “The Yin Yang Symbol”:
The image consists of a circle divided into two teardrop-shaped halves – one white and the other black. Within each half is contained a smaller circle of the opposite color.
The Chinese characters for Yin Yang are 陰陽 / 阴阳 and they are pronounced yīn yáng.
The first character 陰 / 阴 (yīn) means: overcast weather; feminine; moon; cloudy; negative electrical charge; shady.
The second character 陽 / 阳 (yáng) means: positive electrical charge; sun.
The simplified characters 阴阳 clearly show the moon/sun symbolism, since they can be deconstructed to their elements 月 (moon) and 日 (sun). The element阝 is a variant of the radical 阜 which means “abundant”. So Yin Yang could represent the contrast between the full moon and the full sun.
But Yin Yang is much more than just a pair of opposites. It conveys the idea that each of the opposites is dependent on the other, and how they continuously transform from one into the other.
Here is Elizabeth Reninger explaining the meaning of the Yin Yang symbol:
The curves and circles of the Yin-Yang symbol imply a kaleidoscope-like movement. This implied movement represents the ways in which Yin and Yang are mutually-arising, interdependent, and continuously transforming, one into the other. One could not exist without the other, for each contains the essence of the other. Night becomes day, and day becomes night. Birth becomes death, and death becomes birth (think: composting). Friends become enemies, and enemies become friends. Such is the nature – Taoism teaches – of everything in the relative world.