Okay kiddo, so imagine you have a big playground with a bunch of hills, valleys, and bumps. A Jacobi set is like a map that shows you all the places where the ground is flat or doesn't change much.
But instead of using a regular map, which shows you everything all at once, the Jacobi set only shows you the important parts. Like, if you were playing a game of tag, you wouldn't need to know where all the hills and valleys are, you just need to know where the flat spots are so you can run faster.
Scientists and engineers use Jacobi sets to figure out where things are changing the most in their work, like in computer graphics or physics simulations. They can quickly find the important parts of a system, without worrying about all the details that don't matter as much.
So in short, the Jacobi set is like a special map that helps people find the important parts of a system without getting bogged down in all the details. Hope that helps!