From a post I wrote years ago:
Hi Justin Struble we have to be very careful in interpreting that Nibbana sutta. First of all we have to understand what 'Nirvana/Nibbana' means in context. As Ven Hui-feng puts it, "keep in mind the basic metaphorical meaning of the term nirvana, the extinguishing of a flame". The main analogy given by Buddha for nirvana is the extinguishing of a flame. As Ven Nanananda also pointed out,

"Regarding this concept of Nibbàna too, the worldling is generally tempted to entertain some kind of ma¤¤anà, or me-thinking. Even some philosophers are prone to that habit. They indulge in some sort of prolific conceptualisation and me-thinking on the basis of such conventional usages as `in Nib­bàna', `from Nibbàna', `on reaching Nibbàna' and `my Nib­bàna'. By hypostasizing Nibbàna they de­velop a substance view, even of this concept, just as in the case of pañhavi, or earth. Let us now try to determine whether this is justifi­able.

The primary sense of the word Nibbàna is `extinction', or `extin­guishment'. We have already discussed this point with reference to such contexts as Aggivacchagottasutta.[8] In that dis­course the Bud­dha explained the term Nibbàna to the wan­dering ascetic Vaccha­got­ta with the help of a simile of the ex­tinction of a fire. Simply be­cause a fire is said to go out, one should not try to trace it, wondering where it has gone. The term Nibbàna is essentially a verbal noun. We also came across the phrase nibbuto tveva saïkhaü gacchati, "it is reck­oned as `extinguished'".[9]"

Extinction of what? Extinction of passion, aggression and delusion driving the whole mass of samsara. Extinction of the the whole mass of suffering/samsara in the twelve links from ignorance up to old age, sickness and death.

Next is the terms 'unconditioned/death-free/etc' it is very easy to reify this in terms of a metaphysical entity. This is not the case.

Here are some quotations which should hopefully clarify:

Nana/Geoff: "“Firstly, while the translation of asaṃskṛta as “the unconditioned” is fairly common, it’s a rather poor translation that all too easily leads to reification. The term asaṃskṛta refers to a negation of conditioned factors, and the meaning is better conveyed by “not-conditioned.” Secondly, for Sautrāntika commentators, and many mahāyānika commentators as well, an analytical cessation (pratisaṃkhyānirodha) is a non-implicative negation (prasajyapratiṣedha), i.e. a negation that doesn’t imply the presence of some other entity, and therefore nirvāṇa simply refers to a cessation that terminates the defilements and fetters that are abandoned by the correct practice of the noble path. It doesn’t refer to an entity or state that is substantially existent (dravyasat).” "

Nana/Geoff: "One has to be careful with such descriptions which may seem to be pointing to some sort of truly existent "unconditioned ground." Nibbāna is the extinguishment of the mental outflows (āsavā). The liberated mind is measureless (appamāṇa). This is not a "state of oneness with all of existence." It's an absence of identification (anattatā). It's non-indicative (anidassana), unestablished (appatiṭṭha), and not-dependent (anissita). None of these adjectives entail any sort of metaphysical "ground of being" or "unconditioned absolute." They are all negations. An arahant has simply "gone out."

tiltbillings: "There is no "deathless." That is a bad translation leading to an objectification/reification of the idea of awakening. With awakening, there is no more rebirth, one is free from death. (31 words.)""

Loppon Namdrol/Malcolm: “When you have eradicated all afflictions which cause rebirth, this is all the deathlessness you need. No more birth, BAM! no more death.”

Buddha: "And what, monks, is the not-fabricated (asaṅkhata)? The elimination of passion, the elimination of aggression, the elimination of delusion: this is called the not-fabricated. " .... "And what, monks, is the death-free (amata)? The elimination of passion, the elimination of aggression, the elimination of delusion: this is called the death-free." - SN 43 Asaṅkhata Saṃyutta - more in

I can provide many more quotations but this will suffice for now, I think. Nirvana is extinction, like the blowing out of a flame, it is simply and merely the end of suffering and afflictions and does not imply a metaphysical substantial existent as some may postulate. There is no "The Unconditioned" or "The Unborn" or "The Deathless" as some sort of metaphysical essence. There is an unconditioned dharma - analytical cessation (nirvana) - that is the end of birth and death (death-free), is not conditioned (by afflictive causes and manifestations) etc.

All these are classic Nirvana stuff found in the earliest teachings in Pali suttas. In Mahayana emptiness, there is another understanding of "unconditioned" and that is as what Kyle said which I find to be very well said:

"The unconditioned is the emptiness of the skandhas.

Recognition of the emptiness of the skandhas means that the skandhas are non-arisen, what has not arisen cannot be conditioned."

In any case, whether the classical nirvana understanding of the earliest text, or the emptiness understanding of unconditioned/non-arisen, there is no postulating of a truly existing metaphysical essence.

For a more experiential description on what Nibbana is and the relation to the recognition of anatta (selflessness) do refer to the articles I pasted in 
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