The Grim Reaper and the Skull were often used as symbols in medieval and renaissance art as a stark reminder of the fact that life is finite and hints powerfully at the afterlife that awaits us. Many paintings of saints in particular prominently feature skulls, and art that depicted famine and disease often contain the cloaked and hooded figure of the Grim Reaper, skull peeking out, scythe in hand to better harvest human souls.
This is a potent symbol of death and a graphic reminder of the transitory nature of life.