Nibbana is the ultimate goal of the Buddha’s teachings. Among Theravadins it’s generally accepted that nibbana is beyond words and the Buddha himself appears reluctant to give us a detailed description. Nevertheless, in this post I suggest that contemplating what the scriptures say about nibbana serves to reminds us what the ultimate spiritual goal is and can motivate us to strive harder in attaining it…
Nibbana is the ultimate goal of the Buddha’s teachings.  So what is it exactly?
Among Theravadins it’s generally accepted that nibbana is beyond words. For example –
“There’s nothing you can compare it to. No convention can reach it. This is why we say Nirvana has no color. All colors are merely conventions. The state which is beyond the world is beyond the reach of worldly conventions… It is beyond language. You can’t put it into words, you can only talk about ways and means of realizing it.” 
The Buddha himself appears reluctant to give a detailed description of nibbana, but he says enough about it for us to decide whether or not it’s worth striving for –
“This is peace, this is exquisite — the resolution of all fabrications, the elinquishment of all acquisitions, the ending of craving; dispassion; cessation; Nibbana.” 
“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.” 
The Pali canon states plainly that one cannot know nibbana until all craving (arising from ignorance of reality) has been extinguished:
“Bhikkhus, when ignorance is abandoned and true knowledge has arisen in a bhikkhu, then with the fading away of ignorance and the arising of true knowledge he no longer clings to sensual pleasures, no longer clings to views, no longer clings to rules and observances, no longer clings to a doctrine of self. When he does not cling, he is not agitated. When he is not agitated, he personally attains Nibbana. He understands: ‘Birth is destroyed, the holy life has been lived, what had to be done has been done, there is no more coming to any state of being.'”
Since I’m far from enlightened, I find it more fruitful to contemplate the ‘born’ instead of the ‘unborn’.
What is born? Ultimately, that which is born is a conditioned mind state – the result of ignorant clinging to something that is impermanent, unsatisfactory and not-self.
So, for example; I get what I want and thus am I born into gladness. I lose what I cherish and thus am I born into sorrow. This tendency to keep creating worlds and then moving into them is a process called Samsara (literally, ‘wandering-on’). 
But if I truly understood anicca, dukkha, anatta – like the Buddha did – there would be no clinging to what is gained or lost and therefore no birth into gladness or sorrow. I would see that nothing inside or outside of this body/mind is worth clinging to as ‘I’, ‘me’ or ‘mine’. Ageing, sickness and death would be transcended (as when the Buddha experienced severe and fatal food poisoning, for example).
That’s nibbana as I understand it, useful to contemplate as a reminder of the ultimate spiritual goal (escape from samsara) and to motivate one in striving to attain it.
 ‘Nibbana’ edited by Access to Insight. <http://www.accesstoinsight.org/ptf/dhamma/sacca/sacca3/nibbana.html>
 ‘Opening the Dhamma Eye’ by Ajahn Chah. < http://www.accesstoinsight.org/lib/thai/chah/atasteof.html#eye>
 ‘Nibbana’. Ibid.
 ‘Nibbana’. Ibid.
 ‘Cula-sihanada Sutta: The Shorter Discourse on the Lion’s Roar’ (translated from the Pali by Ñanamoli Thera & Bhikkhu Bodhi). <http://www.accesstoinsight.org/tipitaka/mn/mn.011.ntbb.html>
 ‘Samsara’ by Thanissaro Bhikkhu. <http://www.accesstoinsight.org/lib/authors/thanissaro/samsara.html>