Okay kiddo, so you know how when someone has a really important job or responsibility, they might feel like they have a heavy weight hanging over their head all the time? That's kind of what the Sword of Damocles refers to.
In the song by Rufus Wainwright called "Sword of Damocles," he's talking about how it feels to have a lot of pressure and stress on him all the time. He sings about how he's always worried about failing or letting people down.
The story behind the Sword of Damocles is that there was a man named Damocles who was jealous of a king named Dionysius. Dionysius wanted to show Damocles what it was really like to be a king, so he invited him to sit on the throne for a day.
But while Damocles was there, Dionysius had a sword hanging over his head by just a single hair. This made Damocles realize that even though he was in a really powerful position, there was always the risk of something bad happening.
So when Rufus Wainwright sings about the Sword of Damocles, he's saying that he feels like something bad could happen to him at any moment, even though he's successful and has a lot going for him. It's a way of explaining how stress and pressure can weigh on us, even when things seem to be going well.