Okay kiddo, imagine you are playing a game with your friends and you are the boss of the game. Plenary authority is like being the boss in charge of a really important decision.
Let's say you want to build a treehouse and you are the boss of the treehouse project. You have plenary authority to make all the decisions about the treehouse since you are in charge. You get to decide how big the treehouse should be, what color to paint it, and who gets to come inside.
Just like you have the final say in your treehouse project, plenary authority means that the person in charge (like a boss or a leader) has the ultimate power to make decisions and take actions without needing approval from anyone else.
It's kind of like being the boss of the entire game you and your friends are playing. That is a really big responsibility and requires being careful and thoughtful about making decisions that work for everyone!