The single circle at the top of the Taijitu Shuo represents wuji - undifferentiated timelessness. What we see below that is actually an early version of the Yin-Yang Symbol - and represents the first movement into duality - the play of Yin Qi and Yang Qi. From the blending of Yin Qi and Yang Qi come the Five Elements: Earth, Metal, Water, Wood & Fire. From the Five Elements are born the "myriad things" of the world.
Taoist practitioners enter into a "path of return" - a movement from the myriad things of the world back into wuji. The Immortals, or those who have entered the Tao, are those who have completed this "path of return."
"Through practice I came to understand that love is the source of all - love that is unconditional and selfless: love which is totally free. Qi came into being, flowing out of unconditional love. From timelessness, from wuji, qi created the universe. From a non-definable reality, yin and yang, the world of duality, came into being. Wuji became taiji. Yin qi and yang qi blended together and gave birth to the universe. It is qi that created the universe and it is unconditional love that gave birth to qi."
~ Lu Jun Feng, Sheng Zhen Wuji Yuan Gong: A Return To Oneness