A plenary session is like a really big meeting where a lot of people get together to talk about something. Just like how you might have a meeting in your classroom with your teacher and your classmates, a plenary session is like that but with a lot more people, like when all the students in your school meet together in the auditorium. At a plenary session, people might talk about important things like making new rules or decisions, or they might listen to someone who knows a lot about a particular topic give a presentation. Sometimes at a plenary session, people vote on things, like you might have a class vote about which game to play during recess. A plenary session is a way for a lot of people to come together, share their ideas, and make important decisions together.