The Sanskrit word Prajna means consciousness. The consciousness is Brahman. This is the meaning of this Mahavakya. According to Guru Gita, it is Upadesa Vakya, the statement of instruction or guidance. Some call this Mahavakya as Lakshana Vakya, the statement that depicts the nature of Brahman. Etymologically, Pra means Supreme. Jnana means Knowledge or wisdom or awareness or consciousness. We may also take the meaning as the supreme knowledge is Brahman.
Who am I? I am not the body. I am not the mind. I am not the sense organs. If I go on negating like this, only the consciousness remains. That consciousness is Brahman. This is the meaning of this Mahavakya.
Chapter three of the Upanishad contains only four verses.
Om̃ ko’yamātmeti vayamupāsmahe kataraḥ sa ātmā । yena vā paśyati yena vā śṛṇoti yena vā gaṃdhānājighrati yena vā vācaṃ vyākaroti yena vā svādu cāsvādu ca vijānāti ॥ 1॥
Who is He whom we worship thinking: ‘This is Atman?’ Who is Atman? Is it He by whom one sees the form, by whom one hears the sound, by whom one smells the odor and fragrance, by whom one speaks the speeches and by whom one tastes the sweets and the rest?
yadetaddhṛdayaṃ manaścaitat । saṃjñānamājñānaṃ vijñānaṃ prajñānaṃ medhā
dṛṣṭirdhṛtimatirmanīṣā jūtiḥ smṛtiḥ saṃkalpaḥ kraturasuḥ kāmo vaśa iti ।
sarvāṇyevaitāni prajñānasya nāmadheyāni bhavaṃti ॥ 2॥
It is that known as heart, mind, consciousness, discrimination, wisdom, inference, perception, firmness, idea, acuteness, quickness, memory, volition, decision, strength, desire and control. All these are indeed the various names of consciousness.
eṣa brahmaiṣa indra eṣa prajāpatirete sarve devā imāni ca pañcamahābhūtāni
pṛthivī vāyurākāśa āpo jyotīṃṣītyetānīmāni ca kṣudramiśrāṇīva ।
bījānītarāṇi cetarāṇi cāṇḍajāni ca jārujāni ca svedajāni codbhijjāni
cāśvā gāvaḥ puruṣā hastino yatkiñcedaṃ prāṇi jaṅgamaṃ ca patatri ca
yacca sthāvaraṃ sarvaṃ tatprajñānetraṃ prajñāne pratiṣṭhitaṃ prajñānetro
lokaḥ prajñā pratiṣṭhā prajñānaṃ brahma ॥ 3॥
This Brahman, this Indra (the king of the gods), this Prajapati (the creator), all these gods, Pancha Bhutas or five elements namely earth, air, ether, water, and fire, these small beings, these other beings, the seeds of generation, these egg-born, these womb-born, these sweat-born, sprout-born, horses, house, men, elephants , whatever else that breathes, flies, moves, and is immovable; all these are guided by consciousness and supported by consciousness. The universe has consciousness as its guide. Consciousness is the basis. Consciousness is Brahman.
By means of this consciousness, Atman flies from this world, obtains all desires in heaven and becomes immortal and immortal forever.