Arjuna's last question was about death, and how to remember the Lord at the time of death.
In response to that the Lord explained many things right from Sloka 5.
Now in these Slokas we are seeing now, The Lord explains the ephemeral nature of the worlds. And while doing so he actually is explaining the impermanance of 'Life'. Even the longest of lives, as that of Brahmaji is only relative. As Gurudev puts it, life is nothing but a 'shuttle between the un-manifested and the manifested'.
Brahmaji is the Creator, from whom all beings have evolved. Even Brahmaji's duration is not permanent says the Lord, what to talks about jivas?
Brahmaji is said to live for 100 years. There are 4 Yugas - Satya, Treta, Dwapar, Kali. One Maha Yuga is a thousound cycles of these 4 Yugas . So when one Maha Yuga (comprising of the 1000 cycles of these 4 Yugas) has passed in "earth time", then one day of Brahma is gone. Similar is his night. In this way he lives for 100 years. Now we can caluclate in "earth time" , what is the life span of Brahma jii !!!
This long life span of Brahmaji is also not permanent says The Lord. Man always tries to cling to life, prolong it, but Brahma ji himself cannot stay forever. We may from the standpoint of our equipment feel Brahma ji lives long. But if one asks Brahma ji he will say it is too short. An ant feels we live very long, but if the "ant" asks us we will not agree.
This is because the concept of Time is relative, which is also the theory of Einstein. Time appears long or short as per the equipment with which we percieve change. It is common experience even within a human perception, that time goes fast when we are happy and goes slow when our mind is in sorrow. Therefore we use terms like "time flies". So the fact that the concept of Time is relative is our own experience also. Therefore, Brahma ji being higher levels of Consciousness and more happy beings, time for them runs fast, and therefore does not appear so long as it may appear to us from the earthly plane. Bhagavan Sri Krishna is only explaining to us, an idea of Brahma ji's life span from our earthly realm standpoint. Was he to explain this Sloka to members of Satya loka, the time scales in comparison to Brahmaji's day and night would have been different! For as we already said Time is relative, it depends on the equipment with which we perceive change.
During the coming of Brahma ji's day all beings come out. Brahma ji is the Creator or the Total Mind . Therefore at the start of the day Of Brahma ji , Creation happens. Then at the end of a Maha Yuga, Pralaya happens, and all the manifested beings and objects go back to their source , the Creator - this happens in his "night". They then stay there in dormant unmanifested condition, till Brahma ji wakes up, and in the coming of the next day of Brahmaji creation again happens. Creation therefore is only cyclic .
Here in this Sloka the Lord says that the jivas, because of their own impressions and karma phala are helplessly born at the coming of the day of Brahma ji (meaning they become one in the created). And again at the end of the MahaYuga they go back to seed form in the night of Brahmaji, just waiting for the nexy day of Brahmaji to come out again, helplessly.
The Lord uses the word 'avasaha' , helplessly. Again and again we have been born and have died, yet we cling to the impermanent. Therfore he uses this word to show that though we do not want this cycle of birth and death, yet because of ignorance and attachment we continue to do so.
Therefore, now in the next Sloka (Sloka 20), the Lord talks about the substratum of even Brahma ji, that plane of Consciousness which does not take part in this continous cyclic process of manifestation and unmanifestation. That is called as Bhagavan or Isvara, and becoming one with Him one does not take part in this endless painful drama of manifestation (birth) and un-manifestation (death).
That Isvara or the highest plane of Consciousness , the Lord will say is only attained by unswerving devotion. This attainment of the highest plane of Consciousness is liberation.