The one verse from the Upanishads that most informed my perspective on the world is probably this one, from my favorite Upanishad, the Taittiriya. I’ve abridged it slightly for brevity.
“Om. The Brahman-knower obtains the Supreme. As has been said: he who knows Brahman as Satya (Truth or Reality), as Gnyana (Knowledge), and then as Ananta (Unending, Infinite, Eternal), placed in the secret cave of the heart and in the highest heaven, realizes all desires along with Brahman. From this Atman verily arose Akasha (space or ether, a fine and subtle substance permeating the universe); from this came air, from air fire, from fire water, from water the earth, from the earth herbs, from herbs food, from food Purusha, the ‘person’. … From food (anna) verily are produced whatsoever creatures dwell on the earth. By food alone they live. And at the end they pass into it. Food is verily the first-born of the being… Verily those who worship Brahman as food obtain all food.
“Different from, and within that sphere that consists of food (Annarasamaya Kosha or Annamaya Kosha) is the Atma Pranamaya (‘self that consists of life-breath’) by which it is filled. This has the form of a Purusha… the Devas (demigods) breathe this Prana, as also do men and beasts; Prana is the life (Ayus) of all. They who worship Brahman as Prana attain a full life… this is the Atman of the former.
“Different from and within the Prana-sphere is the Atma Manomaya (‘self made of mind’), by which it is filled. This has the form of a Purusha…
“Different from and within it is the Atma Vignyanamaya (‘self that consists of understanding’), by which it is filled. This too has the form of a Purusha. Shraddha (faith) is its head, Rita (order) its southern side, Satya (truth) its northern side, Yoga its Atma, and Maha (the great one) its foundation. Understanding directs the Yagnya (fire sacrifice) and the actions. All Devas worship the Brahman which is understanding as the foremost…
“Different from and within Vignyanamaya is the Atma Anandamaya (‘self consisting of bliss’), by which it is filled. This too has the form of a Purusha … His body is the bliss (Ananda Atma), Brahman the foundation… He who is here in the Purusha and yonder in the sun, he is one. He who knows this, on departing from this world, reaches the Atman consisting of food, of life-breath, of mind, of understanding, of Anandam. Where words do not reach and the mind cannot grasp, there is the Brahman full of Ananda; who knows it does not fear anything.”
This passage is one of the primary sources of my perspective on the nature of this world. The five strata comprise the nature of both Atman and Brahman. Each reality is the inner core of the one preceding, and the very heart of reality is Anandam. The body needs food, which builds up the material body; it requires prana, which builds up the subtle spiritual body; it requires mental contents, which builds up the mind; it requires understanding, which develops deep insight; but the core of reality is pure joy.
The nature of each Kosha, or sheath, is further illumined by extension of the relationships between food and its corresponding Kosha specified by the early part of the passage. “From food verily are produced whatsoever creatures dwell on the earth. By food alone they live. And at the end they pass into it.” Likewise, according to the Vaidik tradition, from Prana are produced those otherworldly creatures who dwell in the Pranamaya sphere, ethereal and elemental spirits whose bodies ingest various types of prana and are composed of it just as our bodies ingest and are composed of various types of physical food. From Manas, Mind, are produced those creatures that dwell in the Manomaya sphere – thought-forms and mental and emotional constructs. And so on up to the Anandamaya sphere, whose dwellers are comprised of pure joy – the great Gods themselves.
Sanatana Dharma traditionally teaches spiritual practices focused on each of the five Koshas. Those that work with the physical body are obvious – Yoga asanas, sacred dance, martial arts, etc. The traditional practices of Pranayama, Yogic techniques and exercises using the breath, work directly at the level of the Pranamaya Kosha. The chanting of Mantras and holy scriptures is a traditional set of practices that work in the Manomaya Kosha, though many Mantras are designed to affect other Koshas as well. Meditation is the traditional practice that works in the Vignyanamaya Kosha. And devotional practice is the traditional sadhana, or spiritual practice, that reaches and stimulates the Anandamaya Kosha. Many practices, such as those stimulating great power centers like the Chakras or the Kundalini, touch upon multiple Koshas. The navel is said to be the main vital center for the Annamaya Kosha, but it is also a major locus of the Pranamaya Kosha. The heart is the main center for the Pranamaya Kosha, but obviously it is of indispensable importance to the Annamaya Kosha, and its importance returns again in the Anandamaya Kosha. The head is the main center for the Manomaya and Vignyanamaya Koshas.
The five Koshas correspond to a slightly simpler Vaidik model of the body, the three Shariras. The Annamaya Kosha is also known as the Sthula Sharira, the body made of physical matter. The Pranamaya, Manomaya, and Vignyanamaya Koshas together comprise the Sukshma Sharira, the “subtle energy” body. The Anandamaya Kosha is also known as the Karana Sharira, the Causal Body, the transcendent cause that underlies and gives rise to all grosser manifestations, and to which the consciousness evolved through those manifestations will ultimately return.