Okay, kiddo. Have you ever heard the phrase "mission accomplished"? It's when someone completes a goal or job that they were assigned to do.
Well, a chargé de mission is someone who is in charge of helping to accomplish a specific goal or project. They work with a team to make sure everything is organized and the job is done correctly. It's like a team leader, but they have a fancier title.
For example, let's say you and some friends want to build a fort in the backyard. You could be the chargé de mission and make sure everyone knows what materials to bring, how to put it together, and when it needs to be finished by. You would help coordinate things so that the fort is built successfully.
So basically, a chargé de mission is someone who helps lead a group to get a specific task done well.