Okay, so a plenary council is kind of like a big meeting that happens in the Catholic Church. Imagine if all the Catholic leaders from different parts of the world came together to talk about how to be better Catholics and how to help people be better Catholics. That's what a plenary council is.
During a plenary council, the leaders talk about things like how to pray, how to be kind to others, and how to make sure everyone in the church is following the rules. They might also talk about new ideas or ways to make the church better.
Basically, a plenary council is a very important meeting where Catholic leaders discuss how to be better Catholics and how to make the Catholic Church a better place for everyone.