Sacramentum Poenitentiae is a special thing people do in the church when they want to say sorry for something they did wrong. It's like going to the doctor, but instead of physical sickness, it's for the things you did that broke the rules or hurt somebody else's feelings. When you go to the church to do Sacramentum Poenitentiae, you talk to a person called a priest, who will listen to what you have to say and help you feel better by giving you advice and telling you how to make up for what you did. It's important to do Sacramentum Poenitentiae because it helps you feel better, and it lets you know that God loves you and forgives you no matter what you did wrong.