Qi prounounced CHI
Acupuncture describe Qi as the energy running through the body like an electric current.
In Chinese philosophy (origins of Acupuncture) Yin and Yang are polar opposites. Yin is negative and Yang is positive. Yin is feminine and Yang is masculine. Yin is dark and Yang is bright. The small circles inside indicate there must be a little yin in yang and yang in yin.
When Yin and Yang are balanced and Qi is running smoothly through the body, harmony exists.