avyaktokshara ityuktastamaahuhu paramaam gatim |
yam praapya na nivartante taddhaama paramam mama || 21 ||

He who is called unmanifest and imperishable, he who has been spoken of as the supreme goal; having attained him (beings) do not return, that is my supreme abode.
avyaktaha : unmanifest
aksharaha : imperishable
iti : in this manner
uktaha : has been called
tam : he
aahuhu : spoken
paramaam : supreme
gatim : goal
yam : he who
praapya : having attained
na : do not
nivartante : return
tat : that
dhaama : abode
paramam : supreme
mama : my
Earlier in this chapter, Shri Krishna had mentioned that those who attain Ishvara are not subject to further rebirth. In this shloka, he says that the “another unmanifest” that remains unaffected by the day and night of Lord Brahma is nothing but Ishvara. He also mentions the nature of Ishvara as imperishable, unmanifest and supreme.
With this shloka, we come back to the main theme that began in chapter seven – Ishvara. After having explained that this universe is subject to cycles of creation and dissolution, and that unless we take effort, we are stuck in this infinite cycle, Shri Krishna reiterates the need for the pursuit of Ishvara as the means of liberation.
So therefore, for the seeker who performs karma yoga and upaasanaa or devotion towards Ishvara attains Ishvara after he has completed his time on earth and in the abode of Lord Brahma. Shri Krishna summarizes the means of attaining Ishvara in the next shloka, which also concludes the topic of liberation from rebirth.