This is a philosophy as well as a martial art technique. (The correct spelling is “Taiji” but “Tai Chi” is more common.)

The philosophy of Tai Chi comes from Taoism. According to Taoism, the essence of life has no limitation. In Chinese it’s called wuji.

According to the philosophy, wuji acts in two opposite directions: hard/soft, wet/dry, cold/hot, slow/fast. One direction is called Yin, which is hard, wet, cold, and slow. The other one is Yang, which is soft, dry, hot, and fast. When you can harmonise these two opposite forces, you are living the philosophy of Tai Chi.

In the martial art sense, Tai Chi is when you are able to freely control these two strengths at the same time.