We can only talk about one in comparison to the other. Yang ascends, goes from the inside towards the outside. It's warm, light, active, protecting, day, summer, fire etc. After a while Yin becomes Yang and Yang becomes Yin, like day becomes night, summer becomes winter and action becomes rest. Health in this philosophy means a balance between Yin and Yang.
Illness means that one of the two is too strong or too weak.
With acupuncture and herbs, the weak pole can be strengthened and the strong pole can be inhibited. Qi circulates through energy channels in the body which are called meridians. Most meridians bear the name of the organ related to it. For example there is a Liver channel, Heart channel etc. These meridians form a complex network in which all energy is distributed throughout the body.