The universe always maintains balance between various forces. God has created everything in pairs, one opposite to the other. This concept was known to the scholars of ancient times, the traces of which can be found in old Chinese culture. They exactly knew that natural equilibrium in universal forces exists at all times and represented it by a symbol in Taoism, which represents universal balance, polarity and complimentary opposites like health/disease, light/darkness, male/female, good/bad, negative/positive and so on.
At one time this symbol was not as well known as it is now. It can be commonly seen as bullets and thumbnails et cetera on websites, on shirts and many other places. I guess that most of such users do not know the origin of this symbol and what it represents. They use it just because it looks nice. Yin-Yang symbol gained popularity after Mrs. Hawayo Takata wore it on her shirt. There is one such picture floating around on Internet showing Mrs. Hawayo Takata wearing a shirt with this symbol printed on front side. Probably she knew what she was wearing and did not use it because of just the appearance.
Yin-Yang symbol does not belong to Reiki taught by Dr. Mikao Usui but many Reiki teachers talk about it during their teaching classes because Reiki restores the balance of energy. The purpose of this article is to highlight main features of what the symbol represents and leave it to the users to decide how to use or not to use this symbol.
Describing the symbol in one word, it represents "equilibrium" or "balance". White and dark portions of the symbol are exactly symmetrical and represent the two primal cosmic forces in the universe. According to Taoism, where there is health there is diseases as well, where there is good there is bad as well and so on. Following are few examples of dual but naturally opposite items:
Similarly the food items, various types of energies and diseases also belong to either yin or yang sides. If you can understand this concept and apply it to practical situations, you can get much better results but you must understand what you are doing and where and why you are using the Yin-Yang symbol. I do not think using any symbol anywhere without understanding is a bright idea.