In ontological dualism, the world is divided into two overarching categories. Within Chinese culture and philosophy, the opposition and combination of the universe's two basic principles are expressed as yin and yang. Some of the common associations with yang and yin, respectively, are: male and female, light and dark, active and passive, motion and stillness. Some scholars believe that the two ideas may have originally referred to two opposite sides of a mountain, facing towards and away from the sun. The yin and yang symbol represents the philosophy of balance, where two opposites co-exist in harmony and can transmute into each other.

Source: Wikipedia