In plain words
At the coming of the day, all manifestations arise from the unmanifest; at the coming of night, they dissolve into that very unmanifest.
Word for word
From the unmanifest all manifest things come forth at the coming of day; at the coming of night they dissolve into that very [one] called the unmanifest.
अव्यक्ताद् व्यक्तयः सर्वाः प्रभवन्त्यहरागमेरात्र्यागमे प्रलीयन्ते तत्रैवाव्यक्तसंज्ञके
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
प्रजापतेः अहनि यत् भवति रात्रौ च, तत् उच्यते —
अव्यक्तात् अव्यक्तं प्रजापतेः स्वापावस्था तस्मात् अव्यक्तात् व्यक्तयः व्यज्यन्त इति व्यक्तयः स्थावरजङ्गमलक्षणाः सर्वाः प्रजाः प्रभवन्ति अभिव्यज्यन्ते, अह्नः आगमः अहरागमः तस्मिन् अहरागमे काले ब्रह्मणः प्रबोधकाले । तथा रात्र्यागमे ब्रह्मणः स्वापकाले प्रलीयन्ते सर्वाः व्यक्तयः तत्रैव पूर्वोक्ते अव्यक्तसंज्ञके ॥ १८ ॥
In plain words
What comes to be in Prajāpati's day, and what comes to be in his night — that is what is said now:
'From the unmanifest' — that unmanifest is simply Prajāpati's own state of sleep. Out of that unmanifest condition, everything manifest — everything rooted and everything that moves, every single creature — comes forth, becomes visible, at the arrival of day, which means at the moment Brahmā himself wakes up. And in just the same way, when night arrives, when Brahmā falls back asleep, every manifest thing dissolves back into that same unmanifest state just described.
Close to the Sanskrit
That which comes to be in the day of Prajāpati, and in the night — that is stated —
'From the unmanifest': the unmanifest is Prajāpati's state of sleep; from that unmanifest, manifest beings — 'they are manifested,' therefore 'manifest' — characterized by the stationary and moving, all creatures, come forth, become manifest, at the arrival of day: 'arrival of day' means the coming of the day, at that time of the arrival of day, that is, the time of Brahmā's awakening. Likewise, at the arrival of night, that is, the time of Brahmā's sleep, all manifest beings dissolve into that very previously-mentioned thing called the unmanifest.