Bones are items dropped by almost all monsters when they die, including the players themselves. The main use of bones on f2p Runescape is for training Prayer. Players can bury bones by simply clicking on them when they are in thier inventory.
Regular bones give 4.5 experience when buried. Players looking to train Prayer quickly should bury all bones they see. Another easy way to train Prayer is to go to areas where players commonly kill monsters that drop bones. In certain areas such as the Edgeville Dungeon, bones spawn.