Sanskrit
अव्यक्ताद्व्यक्तयः सर्वाः प्रभवन्त्यहरागमे । रात्र्यागमे प्रलीयन्ते तत्रैवाव्यक्तसंज्ञके ॥
avyaktād vyaktayaḥ sarvāḥ prabhavanty aharāgame rātryāgame pralīyante tatraivāvyaktasaṃjñake
Translation
From the unmanifested all the manifested stream forth at the coming of day; at the coming of night they dissolve, even in That called the unmanifested.
Annie Besant · The Bhagavad-Gita, translated by Annie Besant (4th ed., 1922) — Wikisource · Public Domain
Explanation
The day/night of Brahma from verse 17 is now given content: manifestation and dissolution. At Brahma’s daybreak, unmanifest existence pours out into the manifest cosmos; at Brahma’s nightfall, all of it dissolves back into that same unmanifest. This is a full cosmological cycle operating one level below the higher unmanifest that verse 20 is about to introduce, and verse 19 restates it once more before that turn.