You might have gone through this word a number of times. Now let us study this word in detail. Before the creation of this universe can we imagine what would have existed? The existing thing was Tao. This is a Korean word whose meaning can be understood as godness or holiness and nothing else. From Tao, chi was created which means energy. From chi, bi-polar energy was created (positive & negative) because for creation of anything, bi-polar forces are required. In ancient books, positive and negative energies resembled to male and female energies. From these bi-polar energies everything was created which has lead to our current existence. In Chinese terminology positive energies are called yang and negative energies are called yin. 
YANG = Positive Energy 
YIN= Negative Energy

Both of these energies flow in harmony through our body to make it a living being. If this harmony gets disturbed, it may lead to disease. Treatment of such diseases requires nothing but harmonization between this yin and yang forces.

Yin energies are directed from earth towards heaven whereas the yang energies from heaven to earth. So these energies intersect our body in a longitudinal fashion to keep each and every cell and tissue of our body in full vitality stage.

Acute problems occur mostly due to yang forces whereas yin energies are responsible for chronic problems.