Okay kiddo, today we're going to talk about something called the "representation theory of diffeomorphism groups." Let's start with some simpler words first.
Do you know what a group is? It's just a collection of things that you can combine in a certain way. For example, your toy cars could be a group - you can combine them by lining them up, or making a pile, or arranging them in a circle.
Now, what about a diffeomorphism? That's just a fancy word for a way to bend and twist things. Imagine you have a rubber band - you can stretch it, you can twist it, you can knot it. Each of these actions is a diffeomorphism.
So the diffeomorphism group is just a group of all the diffeomorphisms you can do to something. For example, the diffeomorphism group of a sphere is all the ways you can bend and twist the surface of the sphere.
Now, what is representation theory? That's just a way of studying how things in a group act on other things. For example, if your group is made of toy cars, then the action might be how they move on a racetrack.
So the representation theory of diffeomorphism groups is all about studying how the diffeomorphisms act on other things. For example, if you have a map of the surface of a sphere, you can use the diffeomorphisms to change the shape of the map. The representation theory of the diffeomorphism group tells you how those changes affect the map.
Does that make sense, kiddo?