Polarity. Yin and yang. Light and dark. Masculine and feminine. One cannot exist without the other. It is within you. It is within the universe. The push and pull to find true balance. You can witness this in something as simple as the weather to something more complex like the ups and downs of a relationship. When it comes to astrology, every planet has both a yin aspect and a yang aspect. One is not better than the other, they just are what they are. We humans too have an internal(yin) aspect to us and an external(yang). These energies communicate and influence each other, just like our environments influence us and we influence them. Within a planet, yin is the more internal and reflective aspect while yang is the more external and active aspect. You need both to find the core of each planet and yourself.