Ancient Chinese experts recognized that there is a natural phenomenon of physical matter that is separated into 2 entities called Yin and Yang.
In nature, the duality of life force (matter and phenomenon) coexist and is represented by the symbol of water and fire. Yin and Yang can simply be explained as 24 hours in a day which is divided into daytime (Yang = fire) and night time(Yin = water) or in a one year cycle of winter (Yin = water) and summer(Yang = fire).
Within Yin & Yang, there are numerous states of energy that are constantly changing which occurs in an alternating fashion, If we are to continue our analogy of a "day', we must realize that there is a morning (middle of Yin and Yang) that starts the cycle "noon'" Yang), "sunset" (middle of Yin and Yang) and finally "night" (Yin).
There is a transformation of states between Yin and Yang such as the opposition of Yin and Yang, the interdependence of Yin and Yang, the mutual of Yin and Yang and the inter-transformation of Yin and Yang. This inter-translation is translated into the human body as "Qi" (pronounced Chi - Yang) and blood fluid - Yin. Should the balance become "broken" in the human body. it can develop ailments and/or diseases. The treatments utilizing acupuncture, moxa, cupping and other methods are used to bring the body into balance to heal the body.