Okay kiddo, imagine you and your friend are playing a game outside. You both have different ideas on how to play the game, and you can't agree on the rules. That's where the grown-ups come in. In the grown-up world, when people can't agree on how to deal with a problem, they might have to go to court.
Kompetenz-kompetenz is a big word that means "the power to decide who has the power to decide." This means that when people have a disagreement about something and they go to court, the court has the power to decide who gets to make the final decision about the disagreement.
In our game example, if you and your friend went to court to decide on the rules, the court would have the kompetenz-kompetenz to say who gets to decide the rules. Maybe the judge would say that you get to make the rules because it was your idea for the game, or maybe the judge would say that you and your friend need to compromise and come up with the rules together.
So, kompetenz-kompetenz is just a fancy way of saying that when people have a disagreement and go to court, the court gets to decide who gets to make the final decision.