This is one of the questions the Buddha refused to answer, and if the Buddha refused to answer it, then I'm not going to answer it either, because I don't know.
He didn't describe what it was. He described what it wasn't. He only described it in negatives: unborn, unoriginated, undying, etc. A place where nothing is born and nothing dies.
But he also stated that it was the ending of all Dukkha and the fading away of greed, hatred and ignorance.