In the study of Chinese medicine, we are well when the Yin and Yang of our body is in a state of balance. But when the body body goes out of balance or when when there is too much Yin or Yang, we fall sick. The aim is to keep the body in balance. This is the basic Yin Yang concept.
This is why watermelon is consumed in Summer while double boiled soup is taken in winter. Watermelon has a cooling effect and it serves to cool the body during the hot summer months. In Winter, the hot double boiled soup provide balance by warming the body.
But what has this to do with Feng Shui?
Just like our body, we are also striving for balance in our home. For example, the concrete structure of your home is Yang while the plants in the garden is Yin. A good Feng Shui home should ideally have a mix of concrete structure and landscaping.
Here are more example. A large home with very few occupants is considered to be Yin. A house that is brightly lit and painted entirely in white is too yang. A home that is dark with small doors and windows that are closed most of the time is too Yin and considered to be out of balance.
The balance of Ying and Yang is a fundamental requirement in Feng Shui.
This Yin Yang concept is pervasive in Chinese metaphysics and is use in many other disciplines. Let me give you an example from Chinese naming.
In this system we are influenced by elements that are derived from our date of birth. By charting our birth chart we can see the mix of elements that we are 'made' of. Let's say that you have too little water elements in your chart. Water is needed to restore balance. In the system of Chinese naming, one of the technique is to introduce the character 'water' or either characters that represent water such as river, lake, sea etc into the name.
This is the basic yin yang concept.