What is the meaning of the phrase How unsteady the head on which the crown sits?

Uneasy lies the head that wears a crown... Henry IV part 2, Act 3 scene 1. Spoken by King Henry. To be king or queen of England was a scary business at one time. Many were murdered , Harold II and Richard III killed in battle, King Charles I beheaded, and so on. Many queens were beheaded (Anne Boleyn, Catherine Howard, Lady Jane Grey, Mary Queen of Scots)