The concept of balance is important in Taoism. Everything in the universe is a balance of opposite forces. Night is balanced by day; summer is balanced by winter.
Often the opposite is necessary for completion. For example, a cup holds liquid because it has solid sides, but it also needs a hollow centre to function. If the entire cup were solid, we couldn’t put liquid into it. The cup must be both substance and space.
Taoism refers to these opposing, balancing elements of the universe as yin and yang. Yin is associated with darkness, yang with light. These are not aspects of good and bad, though; they are merely two sides of the same thing.
You can use your own breathing to experiment with yin and yang. We need air in our lungs in order to live. But to take air in, we need to push air out. We need empty lungs before we can fill them.
Take a deep breath; fill your lungs completely. Hold the breath for a moment. Now try to breathe in without first emptying your lungs. What happened? Describe the experience.