The concept of the Atman is one of the most important concepts in yoga and vedanta. The Atman is our True Self, it is Brahman within the individual. Ultimately the Atman and Brahman are one and the same.
It is a paradox that cannot be fully understood except through gaining knowledge of the Self, a state that can’t be described or understood in words – when you get there you will intuitively know that You Are That.
Anyway, this being a more complex concept, there are quite a few books and yogic scriptures which seek to explain qualities of the Atman in terms we can understand with our limited intellects. One of my favorites is repeated at the end of every arati (ceremony of light):
om na tatra suryo bhati na chandratarakam; nema vidyuto bhanti kurto’yamagnih; tameva bhantamanubhati sarvam; tasya bhasa sarvamidam vibhati.
It’s from the Kathopanishad 2.2.15 and it means:
The sun does not shine there, nor do the moon and the stars, nor do lightnings shine and much less this fire. When He shines, everything shines after Him; by His light, all these shine.
In other words, within each of you there is a Self-luminous Atman, it does not need anything at all outside of itself to Be – within you is the whole universe, needing nothing to complete itself.
According to Swami Sivananda (who says it best):
The Atman is self-luminous. It shines by itself. A self-luminous thing is that which is not in need of any extraneous light for its own shining or effulgence. The sun, moon, stars, lightning and fire borrow their light from the Atman. The sun cannot illumine the Atman but it gets its light from the Atman, just as water borrows its heat from contact with the fire. Water does not possess heat as its inherent attribute. From the various kinds of light which shine after Him, the self-effulgent nature of Brahman is inferred. ~ Swami Sivananda
In the Bhagavad Gita 15.6 is another similar verse:
Neither doth the sun illumine there nor the moon, nor the fire; having gone thither they return not; that is My supreme abode.
Here Lord Krishna is explaining to Arjuna that Brahman exists everywhere for all time and needs nothing besides Itself, there is nothing besides Itself.
There are several other scriptures with the same idea (such as in the Svetasvara Upanishad and the Mundaka Upanishad).
Your true nature is divine.