What does it mean to have yin and yang in the theory of yin and yang in TCM?

Yin and Yang are two opposite material properties in nature. For example, day and night, the sun is sunny and the night is cloudy. As for what is the combination of Yin and Yang, that is to say, day and night belong to Yang, relative to night, only for the morning and afternoon (Yang), which belongs to Yang in the morning and Yin in the afternoon. This is the so-called combination of yin and yang.