Okay kiddo, you know how sometimes, when you're playing a game, you have points or levels that go up when you do something good? It's like when you get a high score on your favorite game.
Now imagine that there's a character in a show who is really strong, and they have their own level that goes up when they get even stronger. The number they use to show that level is called a power level.
So when someone says "it's over 9000!", they mean that the character's power level is really, really high. It's like they've reached a level that's almost impossible to reach, and that makes them super strong.
People like to use this phrase as a way of saying that something is amazing or impressive, because when a character's power level is over 9000, it means they're basically unbeatable.