Tai Chi, or taiji, is an ancient in internal martial art and technology of biomechanics, to awaken, mobilize, harvest and groom chi. It is a practice. There is a wealth of steeped Chinese Taoist philosophy and lore associated with it, such as five elements, (metal, water, wood, fire, and earth), wuji, yin and yang, and the foundations of Chinese medicine (meridians, and acupuncture). However, today Tai Chi has no nationality and is international.
I am not an expert in these matters of Chinese philosophy, but will attempt to provide a summary of my understanding and references to other more authoritative sources.
Wu Ji is a resting state, which unifies all elements. The beginning of movement changes the state to Tai Ji, which differentiates into Yin and Yang. The symbol above is ancient and represents Yin and Yang as opposites of each other. They are not symmetric opposites, however. Yang is action, Yin is passive context. Yang is seen as light or positive, Yin as dark or negative. Yang is seen as male and Yin as female. The ways in which Yin and Yang interact gives rise to the five elements of metal, water, wood, fire, and earth. They are arranged in an order that follows one another. Here are some further references for the interested visitor.