My translation of Chapter 6 of Tao Te Ching:
The state of eternal existence
arising from profound female procreation
And the door to this feminine profundity
is the root of Heaven and Earth
Its presence is subtle
Its procreative power has no end
As long as there is human culture, there will be people in search of immortality, eternal existence or rules of profundity. Laozi conceptualized it as continual female procreation that bred Heaven and Earth; and sustains their forever existence. As applied to human society, it is called Tao and Te (morality). And as applied to human mind-body existence, it is called Chi or Qi. Hence the practice of Qi-meditation forms the foundation to a complete understanding and appreciation of Laozi's Tao Te Ching.
Likewise when meditation or Qigong is practiced without regard to the teachings of Tao Te Ching (or other teachings on Tao or morality), it is only a form of mind-body exercise, and has nothing to do with philosophy, practical guidance to good human behavior and spirituality.
The metaphor of female procreation has been interpreted as a sublimation of sexual energy in classical Neidan or Taoist alchemy. The root is in Tao Te Ching itself.