Link to purport by A C Bhaktivedanta Swami Srila Prabhupada

Transcript of Bhakti-Shastri class on this verse by Chaitanya

Then next verse continues the theme, what happens when the day of Brahma ends:

Bg 8.18

avyaktād vyaktayaḥ sarvāḥ
prabhavanty ahar-āgame
rātry-āgame pralīyante
tatraivāvyakta-saṁjñake

Word for word: 

avyaktāt — from the unmanifest; vyaktayaḥ — living entities; sarvāḥ — all; prabhavanti — become manifest; ahaḥ–āgame — at the beginning of the day; rātri–āgame — at the fall of night; pralīyante — are annihilated; tatra — into that; eva — certainly; avyakta — the unmanifest; saṁjñake — which is called.

Translation: 

At the beginning of Brahmā’s day, all living entities become manifest from the unmanifest state, and thereafter, when the night falls, they are merged into the unmanifest again.

avyaktād vyaktayaḥ sarvāḥ, so actually from unmanifest, sarvāḥ all living entities, vyaktayaḥ and prabhavanty ahar-āgame, ahar-āgame is the start of the day and then rātry-āgame pralīyante, that when the night comes pralīyante, destroyed. tatraivāvyakta-saṁjñake, this is referring to the cycle of cosmic manifestation and unmanifestation. 

So we have in physics the law of conservation of energy so matter is considered to be indestructible it will be convertible to one form from another may be it is convertible to energy but convertibility is not the same as destructibility so this principle is even accepted in vedic scriptures so the whole substance of matter that is eternal that is indestructible but actually what we are attracted to in the material world is not the substance but we are attracted to the forms. For example in some traditions when a person dies they put the body on some tower at top and birds come and eat it that is the way they consider that the body can be utilised as food for others. So in this case say bird has eaten the body and it discharges the refuse and the refuse falls down and if someone tells somebody this is actually your girl friend …the other person says …WHAT !!! so someone’s girl friend or boyfriend dies and the body gets converted  to stool then he will be  shocked by that but we are attracted and captivated by the form of the opposite sex, why is that? Because our attraction is to the form and not to the substance. Substance may be the same but we are attracted in the material world by the form. And that is why in the material world when we attached with the form we suffer. So Krishna is telling over here the substance is eternal but from the substance sometime the form is manifest and sometimes it is unmanifest, so at the beginning of Brahma’s day the form become manifested at the end of Brahma’s day the from becomes unmanifested. This way there is a cycle which goes on and on and on, and this goes on for the entire lifetime of Brahma.

End of transcription.