Okay kiddo, have you ever played a game where somebody decides the rules as they go along? Like when you're playing tag and somebody says "No touching the tree!" even though that wasn't agreed upon before?
Well, a kangaroo court is kind of like that but in a more serious way. It's like a fake court where the people in charge don't follow the rules of a real court. They make decisions that don't make sense or aren't based on evidence.
It's not a good thing because it's not fair. Just like in your game of tag, if somebody changed the rules in the middle, you would feel mad or confused. In a kangaroo court, people can get hurt or punished unfairly.
It's important to always try to be fair and follow the rules, especially when we are making decisions that affect other people.