In Nethack, a bones pile is the pile of equipment left behind by a dead character. There is a possibility of another player finding it, but take note that much of it will be cursed. Bones piles can be an excellent source of useful stuff, but they can also be dangerous, because you will have to deal with whatever killed the former player. It is fairly common for people to be killed on the same bones level multiple times, especially if it contains a monster with a wand of death or tough feral pets.