What was born first from ascetic heat, that was born before the waters — entering the cave and remaining there, that was seen through the elements. This is in truth That.

This verse describes Brahman as the primordial principle.

Yaḥ pūrvaṃ tapaso jātam — what was first born from ascetic heat (tapas). The cosmic act of self-limitation.

Adbhyaḥ pūrvam ajāyata — was born before the waters. Precedes the elements.

Guhāṃ praviśya tiṣṭhantam — entering the cave and remaining. Guhā is the cave of the heart where the Ātman resides.

Yo bhūtebhir vyapaśyata — that was seen through the elements. Brahman reveals itself through manifestation.

Etad vai tat — this is in truth That. The identity between the cosmic principle and the individual.

This profound teaching indicates that the Ātman in the heart is the same cosmic Brahman.