A framework of wood or metal placed over the coffin or tomb of a deceased person, and covered with a pall; also, a temporary canopy bearing wax lights and set up in a church, under which the coffin was placed during the funeral ceremonies.
Nethack sometimes saves the level on which you die in a "bones file". These files get loaded into later Nethack games. If you're the only Nethack player on your system, you'll only get bones files you created yourself. With Hearse, you can automatically exchange bones files with other Nethack players.
Seeing a hearse predicts a lifting of your burdens. If you rode with the driver the dream indicates an increase in responsibilities, but if you were inside the hearse you can expect to soon make a change which will be important to your future.