Okay, so you know when you eat food, your body breaks it down and turns it into poop, right? Well, sometimes people do things that they feel bad or guilty about, like being mean to someone or not doing their homework. And when they feel bad about it, they might want to 'repent', which means they want to say they're sorry and try to make things right.
One way some people do this is by sitting on a special kind of chair called a "stool of repentance". It's not really a stool, like the kind you sit on to reach high places, but more like a little bench or platform. When someone sits on the stool of repentance, they might talk to someone they trust, like a priest or a parent, and say what they did wrong and why they feel bad about it.
The idea is that by talking about their feelings and trying to make amends, they can start to feel better and maybe even learn from their mistakes. So the stool of repentance is kind of like a way to ask for forgiveness and try to work through your feelings of guilt or shame.