Yin and Yang are two important elements of Feng Shui.
Yin and Yang are opposites that cannot exist without one another and influence each other constantly. Examples in daily life of yin and yang are day and night, man and woman, hot and cold, fire and water.
These opposites create a field of tension that creates movement. When there is too much Yin, Yang automatically appears and vice versa. It is a change of forces and a constant quest for balance. Feng Shui works intimately with these forces.
There are similar energetic tensions in a house. The different functions of the rooms make them more yin or more yang and thus have an influence on the Chi (life energy) flowing through a house. For example, a bedroom is a place where we seek rest which is a very Yin activity. The kitchen is very Yang room seeing we prepare our food here and feeding being a Yang activity.
Feng Shui studies the energy of a given space and seeks to balance the energies to create an environment with optimal life energy and vitality to support the daily life.