It is not something any worldling can know without reaching the culmination point of the noble eightfold path ie.- it is not something worldlings who are wandering in samsara and fully bound to it can ever experience.
It is not part of the samsaric experience, which includes all the planes of existence:
“There is that dimension where there is neither earth, nor water, nor fire, nor wind; neither dimension of the infinitude of space, nor dimension of the infinitude of consciousness, nor dimension of nothingness, nor dimension of neither perception nor non-perception; neither this world, nor the next world, nor sun, nor moon. And there, I say, there is neither coming, nor going, nor stasis; neither passing away nor arising: without stance, without foundation, without support [mental object]. This, just this, is the end of stress.”
— Ud 8.1
However, it is not annihilation either and it is something that exists. That is Nibbana exists:
“There is, monks, an unborn — unbecome — unmade — unfabricated. If there were not that unborn — unbecome — unmade — unfabricated, there would not be the case that emancipation from the born — become — made — fabricated would be discerned. But precisely because there is an unborn — unbecome — unmade — unfabricated, emancipation from the born — become — made — fabricated is discerned.”
— Ud 8.3
Its very existence or to put it in another way, because we can experience something other than phenomena that arise and pass away and is hence suffering, in the Dhamma eye, Fruition (phalasamawata), Cessation (nirodhasamapatti) there is an escape from suffering apparent, while still alive. Without this (non)experience there would not be an escape from phenomena that arise and pass away, that is, conditioned phenomena.
This might seem like annihilation to 1) the worldling that believes there is a self +/- a soul 2) doesn’t accept in rebirth 3) thinks life is sort of ok. In answering this 1) there is only phenomenal experiences arising and passing away- all the ‘truths’ we believe about ourselves is based on this and not something which we don’t experience, which might be ‘out there’. So sticking to what know and base our reality on, -many assumption of what we can know (Self, permanence, the 3D world, substantiality, self-existence) and seen to be false, in vipassana. Nothing is seen to be self, everything is fleeting, more like pixels on a screen rather than a 3D world, insubstantial- phenomena than solid objects, causally arisen. So many aspects of beliefs we hold about the world fall away. Therefore there is no self to be destroyed upon Nibbana- it is merely a process which causes suffering which needs to stop. Also more subtlely, as it doesn’t stop on its own at ‘death’, Nibbana needs to achieved. It isn’t stopping of a ‘wonderful life’ but the end of a ‘wonderful lie’ which keeps giving resulting in suffering. Without seeing the suffering caused by the process, Nibbana won’t be seen to be a good solution as only a small problem is perceived when the real problem is inherent to the system and all encompassing.
Advaita Moksha includes Self (atman) which merges with God (Brahman). These are only philosophical beliefs ie. not really seen by anyone. At best (like all theistic religions) they may be mislabelling of phenomena. The Buddha’s Dhamma doesn’t label anything, but merely describes the container (aggregates, elements, sense doors). That accurate description cuts out many labels that are incorrect, if not impossible. For example, to say God is permanent is impossible because the container that God is in, last only a second. Then something else arises. Therefor the idea of permanence of anything cannot exist- except in delusion (avijja) and projection of that delusion to things which are said to exist ‘out there’.
So the Buddha’s approach to Nibbana is to not project anything on to Nibbana, because whatever we project it doesn’t display the features of… Therefore, views that conceive various thing about Nibbana is considered Wrong:
"He perceives Unbinding as Unbinding. Perceiving Unbinding as Unbinding, he conceives things about Unbinding, he conceives things in Unbinding, he conceives things coming out of Unbinding, he conceives Unbinding as ‘mine,’ he delights in Unbinding. Why is that? Because he has not comprehended it, I tell you. MN1
With metta