It means that "Tao" produces a process in which all things grow from few to many, from simple to complex.
"Tao" is nothingness, and everything is born from nothingness. And Tao generates one, and "one" represents space, the carrier that generates yin and yang. "Two" represents yin and yang. "Three" represents yin, yang, and harmony. "All things" represents everything in the material world.
Tao is unique. Tao itself contains the two qi of yin and yang. The two qi of yin and yang intersect to form a uniform state in which all things are produced.