Okay kiddo, let me try to explain this complicated term in simpler words. Have you ever played a game and when someone wins, they can't play that game again and win? That's kind of like res judicata.
In the legal world, res judicata means that if a court has already made a decision on a case, then the same case can't be brought to court again. It's like the first decision is final and can't be redone.
Let me give you an example. Imagine your friend and you have an argument about who gets to eat the last cookie. You go to your mom and she decides that your friend is the one who gets the cookie. If you go back to your mom again and ask her to make the same decision, she won't do it because she already decided. That's res judicata in action.
So, in the court system, after a judge has made a decision on a case, res judicata means that the case can't be brought to court again with the same issues and the same parties. It helps to prevent people from continuously re-litigating the same issue over and over again.