A headstone is a grave marker, usually inscribed with the dead person's name. You might visit your grandfather's grave each year, leaving flowers beside his headstone.
Another name for a headstone is a tombstone or a gravestone. True to its name, a headstone is often made out of a large piece of stone, frequently slate, granite, or marble. The meaning of headstone was originally "cornerstone," or the stone at the corner of the base of a building, but by the 1700s it came to mean "stone at the head of a grave."