Yin and yang are not opposing forces (dualities), but complementary forces, unseen (hidden, feminine) and seen (manifest, masculine), that interact to form a greater whole, as part of a dynamic system. Everything has both yin and yang aspects as light could not be understood if darkness didn't exist, and shadow cannot exist without light. Either of these aspects may manifest more strongly in a particular object depending on the criterion of the observation. The concept of yin and yang is often symbolized by various forms of the Taijitu symbol, for which it is probably best known in western cultures.
There is a perception (especially in the West) that yin and yang correspond to evil and good. However, in Daoist metaphysics, good/bad distinctions and other dichotomous moral judgments are perceptual and not real, and yin-yang is an indivisible whole. In the ethics of Confucianism on the other hand, most notably in the philosophy of Dong Zhongshu, (c. 2nd century BCE) a moral dimension is attached to the idea of yin and yang.
There's always more to learn about yoga and in turn, learn about yourself.