Death is cessation of physical life. Everybody dies, eventually. A large part of medieval life involved attempting to make other people die, or thwarting their attempts to make you die.
Death was a constant in medieval life. Modern society often insulates us from the reality of death, placing it behind closed doors in hospitals and funeral homes, but for the average person in period, death was a commonplace, even familiar occurence. Even laying aside the constant violence of the medieval age, the lack of medical knowledge and sanitation often meant that disease was rampant.
Personification of Death
Because of the constant threat of death, Death itself became personified in the literature and arts of the day. Death was often depicted as a skeleton, robed in black and carrying a scythe or a sword -- the Grim Reaper still familiar today. A related image was a skeleton clothed in rich robes wearing a crown -- Death as ruler of all things.