na tadbhaasayate sooryaa na shashaanko na paavakaha |
yadgatvaa na nirvatante taddhaamam paramam mama || 6 ||

That in which the sun does not illumine, nor the moon, nor fire, that is my supreme abode. Having attained that, there is no return.
na : not
tat : that
bhaasayate : illumines
sooryaha : sun
na : not
shashaankaha : moon
na : not
paavakaha : fire
yat : which
gatvaa : attained
na : not
nirvatante : return
tat : that
dhaamam : abode
paramam : supreme
mama : my
Shri Krishna listed the qualifications of a seeker in the prior shloka, and asserted that one who takes his refuge will attain his abode. He now provides the location, the address of that abode. He says that there is no sun, no moon, no fire, in other words, no source of light in his abode. Initially we would feel a little frightened if we take the literal meaning of this shloka. Even cavemen were able to access some light source in the form of the sun, the moon, or fire from a wooden torch. Why would anyone want to go to such a place?
The sun, the moon and fire have symbolic interpretations which are extremely relevant here. The sun is the presiding deity of our intellect, the moon of our mind and emotions, and fire of our physiological functions. The one who has sought refuge in Ishvara automatically gives up affinity to his body, mind, intellect, ego (which resides in the intellect) and physiological functions. If this affinity, the root of all our sorrow, is given up, such a person will never again get caught in the wheel of birth of death, in the cycle of samsaara. This is liberation.
So then, this is the abode of Ishvara, of self realization, of liberation. Having reached there, the liberated person does not come back to the state of ignorance. He never gets deluded again. He never identifies or develops affinity with body, mind, intellect and the world. The duality, the pairs of opposites, the dvandva that was mentioned before, is nothing but the world. For such a person, neither joy nor sorrow, neither pain nor pleasure, neither friend nor enemy, nothing can destabilize him. This is liberation.