Sri Aurobindo translates Mundaka Upanishad, Chapter 2, Section 1, Verse 8: “The seven breaths are born from Him and the seven lights and kinds of fuel and the seven oblations and these seven worlds in which move the life-breaths set within with the secret heart for their dwelling-place, seven and seven.”
The Upanishads generally claim to be “the secret of the Veda”, so it is to the Veda that we need to turn when verses such as this arise, and which obviously have some kind of symbolic meaning. In The Secret of the Veda, Sri Aurobindo explores the mystic imagery and symbolism used by the Rishis to describe their inner experiences of their spiritual quest, couched in words that could be interpreted as describing external, physical events or things. In this case, the external forms are very apparently descriptive of an inner state of experience if we are to make sense of “seven worlds” or “seven breaths, etc. The reference to the secret heart as their dwelling place certainly makes it clear that the Rishi is referencing an inner state of consciousness. Sri Aurobindo notes the meaning of the 7 rivers: “We find also that the image of the river or flowing current is used to symbolise a stream of conscious being. We find that Saraswati, one of the seven rivers, is the river of inspiration flowing from the Truth-Consciousness. We have the right then to suppose that the other six rivers are also psychological symbols.”
The mystical experiences and spiritual forces referenced here are all stated to be born from the Brahman. Once again, the primacy lies in the Brahman, not the individual human being who is undergoing the practices of Yoga and the results in the form of various spiritual experiences and realisations. In Yoga texts, we see references to 7 primary chakras, each having their own color and light, each based in a different plane or “world” and having therefore their own various forms of “fuel”. These energies control the life-breaths which reside in the psychic center that joins the individual to the cosmic energies.
Sri Aurobindo, The Upanishads, Mundaka Upanishad, pp. 193-210