Okay kiddo, so imagine you and your friend played a game together and you both did something wrong. But instead of telling your parents or teacher what happened, you two decide to keep it a secret and make up a story to cover up what really happened.
That's kind of like a secret trial. When something wrong happens in the big world outside of just you and your friend, sometimes the government or people who are in charge of making sure everyone follows the rules will have a trial to figure out what happened and who should be punished.
But sometimes, they might decide to keep the trial a secret, which means that nobody knows about it except for a few important people who are involved. They might do this to keep information hidden from others who might want to use it against them, or to protect someone's identity, or to make it easier to make a decision without other people getting in the way.
But just like with you and your friend's secret game, when things are kept secret, it can be hard to know what really happened and if things are fair. So some people think that secret trials aren't a good idea because it's important for everyone to know what's going on and why decisions are being made.
Does that make sense, kiddo?