A plenary session, or "plenarium" for short, is a big meeting where everyone who is supposed to be there gets together to talk about important things. Think of it like a big family dinner, but instead of just talking about what happened that day or what you want for dessert, all the adults are talking about really important things like making rules or deciding what to do about a problem.
At a plenary session, everyone is invited and allowed to speak their mind. It is a fair and open discussion where everyone can share their opinions and ideas. Just like how you might raise your hand in class to share what you know, people at a plenary session raise their hand to share what they think.
Sometimes, people debater about what to do, and there can be different opinions, which means that people do not agree. This is okay, because they can talk about the problem and come to an agreement that works for everyone.
Once everything is discussed, usually a leader or boss, called a "chair," will make a decision based on what everyone said. This decision is like when your mom or dad makes a plan for the family, except it affects many more people.
So, plenary sessions are like really important family dinners where everyone gets to share their ideas, talk about things, and then make a plan for what to do next.