aabhramhabhuvanaallokaahaa punaraavartinorjuna |
maamupetya tu kaunteya punarjanma na vidyate || 16 ||

O Arjuna, all worlds including the abode of Brahma (are subject to) return. But having obtained me, O Kaunteya, rebirth does not occur.
aabhramhabhuvanaat : including the abode of brahma
lokaahaa : all worlds
punaraavartinaha : return
arjuna : O Arjuna
maam : me
upetya : having obtained
tu : but
kaunteya : O Kaunterya
punarjanma : rebirth
na : does not
vidyate : occur
According to the scriptures, there exist heavens or worlds subtler than our visible universe. All beings on this earth who perform meritorious deeds go to one of those heavens after death. The abode of Lord Brahma (Brahma-loka) is considered the highest among the heavens. Shri Krishna says that all beings who end up in these heavens, including the world of Lord Brahma, do not stay there permanently. They have to return to earth at some point. Only those who attain Ishvara gain permanent liberation.
The first half of the shloka is applicable to seekers who perform karma yoga diligently and worship Ishvara as well. Depending upon the sincerity of their deeds and worship, they will attain the appropriate heaven. A select few attain the abode of Lord Brahma which is the highest possible heaven. Here, it is said that the residents only enjoy pleasure. There is no sorrow or suffering whatsoever.
Once they attain the abode of Lord Brahma, they are faced with a choice. They can continue to remain interested in pleasure seeking, or attain liberation. Attainment of Ishvara is the same as liberation. If they continue to remain interested in pleasure seeking, if they think of Brahma loka as yet another realm of space and time, they will eventually come back to earth and start life all over again. If they are interested in liberation, they will attain it when Brahma loka is dissolved along with all of the other worlds. This kind of liberation is called “krama mukti”.
Now, the questions arises, why do all of these worlds end? The topic of cosmic creation and dissolution is taken up next.