The king's jester was Yorick, whose skull Hamlet holds in the graveyard scene in Act V when he says that famous line, "Alas, poor Yorick! I knew him, Horatio." Yorick is important to the play even though he has been dead for "three-and-twenty years" because it is while examining his skull and the bones of other deceased people that Hamlet concludes that death is, indeed, the end of things. After coming to this conclusion, Hamlet no longer fears death.
The king's jester, who is dead, was named Yorick.