You can read a lot about explanation of Yin Yang. Saying Yin is this and Yang is that. Most of the time the explanation is missing why it is like this. Actually the theory is to understand the world or universe or live and its action. If you start to describe something as good you should understand there exist always a opposite. You can describe the night as Yin because you know there is its opposite the day i.e. Yang for example. Same for other things like cold / hot, men / women and so on. Important is to understand that to name something in Yin or Yang the opposite is already included. The Taiji symbol (most people calling it wrongly Yin Yang sign) is showing this understanding very clear. The small spots with its counterpart color within the Yin Yang are showing this deep understanding. If you name something its like one side of a coin it includes the other side of the coin. No good without bad, no happiness without sadness and so on. The Taiji symbol is also showing the dynamic of nature which of course is including our life. Everything is changing to its opposite after it reaches is maximum. Night changes to day life changes to dead one season is changing to the next one are examples of the everlasting processes of the hole universe including our life.
Here some parts of classical texts:
‘When all people in the world know beautiful as beauty, there
appears ugliness; when they know goodness as good, there
appears evil. Therefore by opposing each other existence and
non existence come into being. Difficult and easy form
themselves, long and short are distinct high and low contrast,
sound and voice harmonize front and back emerge.’
Out of Huang Di Nei Jing, the classics of the yellow Emporer:
Huang Di said ‘I hear heaven is yang and earth is yin. The
moon is yin. How are these joined in man (Ling Shu)?’ and
physiology ‘The clear yang forms the skies; the turbid yin the
earth. The earthly energies rise above to form clouds; the
energy of the skies descends as rain. Therefore yang comes from
the upper openings; while turbid yin comes out from the lower
openings. The clear yang develops into the texture of the skin
and the turbid yin runs back into the five Zang (Suwen).
Health preservation and treatment is aimed at establishing
harmony with the universe.