What does it mean to have the sword of damocles hanging over your head?

Damocles and his SWord are figures from Greek legend. Damocles was a courtier who was complimenting a tyrant. The tyrant offered to switch places with him for a day, to which Damocles agrees. The day seems to be going all well for Damocles...until dinner, when he looks up to see a very heavy and very sharp sWord being suspended over his head by a hair. The sWord is symbolic of the constant risk of life the tyrant faces every day because he is king.

If someone says you have the SWord of Damocles over your head, it means your position puts you at constant risk of your life; death can come from anywhere at anytime. Kings and other heads of state always have to watch their backs because being in power makes them targets (it is not hard to look up royal and presidential assassinations). It could also apply more generally to a person in constant fear of being killed (such as an undercover operative in enemy territory).