Yin and Yang pretty much means everything as the whole universe uses these simple rules every second to function, so this section of the site will be dedicated to any articles that are directly to do with yin or yang but also any articles that I could not find a suitable category for.
In tai chi it refers to the constant changing of state in the body and the Qi. At a basic level it's referring to the hands, but it can be used to refer to any part of the body that is changing from one state to the other. The most common use of the word is when you have a relaxed hand, (we call this a yin hand) changing to a flexed hand (we call this a yang hand). Sometimes the changes are more subtle and a little more experience is needed to distinguish whether it's a yin or yang hand, or move. This is the way beginners understand it on a physical level. The hand changes then progress through the whole body, until your body is moving in harmony with your Qi, like a sign wave.
Certain Tai Chi movements or Qigongs can be used to change the bodies state to heal it. For example if the body is too yin. Examples of being too yin would be, not wanting to engage in anything, being very quiet, being very closed off to others. Examples of being too yang would be, being too loud, getting into fights all the time, being very open with emotion, be it angry, upset, even happy. Keep in mind these states could also be caused by factors other than being too yang or too yin.
But if you take a Qigong or Tai Chi form, combined with a change of diet you can change the state of the body for the better. Even if the state is not cause by the condition of being too yang or yin, you can still try out the Qigong or Tai Chi and see if it helps. If it helps then it's probably that the state of body was caused by being either too yin or too yang, and if it doesn't work then no harm will have been done, in most cases though even if the state is not caused by being too yang or too yin, the Tai Chi or Qigong will have helped greatly simply because it helps to balance the body out no matter what the condition.