An appreciation of the concept of Yin and Yang is useful to deepen ones understanding of Tai Chi. Probably the most important thing to notice about Yin and Yang is that they only exist in relation to each other. In the beginning there was wuji. We start the form in the wuji position.
The tai chi symbol represents the interaction of Yin and Yang. The Chinese character for Yin is the dark side of the hill and for Yang the sunny side of the hill.
The taiji symbol shows each of them with a seed of the other within it, to illustrate how, for example, day turns into night and night turns into day.

On the left are some examples of Yin and Yang pairs.

Note that 'standing' is Yang in relation to 'sitting' but Yin in relation to 'walking' which ,in turn is Yin in relation to 'running'.

It is also important not to attach 'good' and 'bad' labels to these aspects or qualities. In Chinese culture balance and appropriate behaviour is the important thing. An illustration of this is an excess of 'joy'. Is it possible to be just too happy? Well next time you see someone laughing at an inappropriate time or too loudly perhaps you'll agree that it is!

Tai Chi is intrinsically concerned with Yin and Yang, the ancient Chinese philosophy of the two forces of expanding and contracting that form the physical universe. In the Yang Family form of Tai Chi each movement is without beginning or end, each move flows seamlessly into the next one, as Yin flows into Yang and back into Yin. Yin and Yang are infinite so you can never achieve the ultimate. Inside each must remain an element of the other and as the extremes are approached so the desire to return becomes greater creating the natural flowing oscillation that is the form.

In the two person forms or in self defence one never meets Yang with Yang as this is the meeting of force with force and will automatically bring hard self defeating force into play. When facing Yang energy the response is to yield with Yin and absorb or deflect until you can respond back with Yang when your partners energy is dissipated.