"Yin and Yang" refers to the alternation of day and night and the alternation of cold and heat in nature. There is also "Shu Shu" instead of "Shu Shu", which has n meanings and can be understood as various health preservation methods of traditional Chinese medicine. This sentence comes from the theory of ancient naivety in Neijing, which means to adapt to the changes of nature and properly use various health preservation methods.