Relief from compulsive grasping, relief from wanting things to show up differently, relief from believing we can arrange things to derive happiness.
Nibbana is generally thought of as an exalted state of being that is free of all suffering, all desire, etc. etc. It, along with confusions arising from the previously mentioned mindlessness schools, leads to people assuming that Buddhism is wireheading. The best translation of Nibbana (for the purposes of practice) IMO is ‘cooling down.’ If we think of the above strategies as a sort of tensing, a sort of effortful exercise, a sort of heating up if you will, then we can contrast it with untensing, non-efforting, cooling down and relaxing. The simplest way to think of this is that Nibanna is the opposite of Tanha. Often translated as the mind ‘inclining towards relinquishment’ (of that which was grasped). The nature of this experience is relief. And here it means not only relief from the particular stimulus that was stressing us out, but the (normally experienced temporarily) relief from compulsive grasping, the relief from wanting things to be other than they are, relief from the belief seemingly pressing down on us that we need to act just for things to be okay. A kind of happiness that comes from a halting of believing that we need to get happiness by arranging things to match up with mental projections.