Okay, so let's imagine that we have a group of little creatures called Kleinians, and they all live in a big magical world called Kleinian space. Now, these Kleinians like to move around a lot, and they can do things like flip and rotate and stretch themselves in all sorts of crazy ways.
Now, imagine that we have a special space within Kleinian space, let's call it S. S is a smooth surface, like the surface of a ball, and it's really important because it helps us understand how the Kleinians move around.
The density theorem for Kleinian groups is basically a rule that tells us how the Kleinians move around on the surface of S. It says that if we take any point on the surface of S, and we look at all the different ways that the Kleinians can move around that point without leaving S, then there are either a finite number of ways or infinitely many ways.
Now, this might seem a bit confusing, but let's break it down. Imagine you're playing with some toys on a table, and you can only move the toys in certain ways without knocking them off the table. If you look at one of the toys and think about all the different ways you can move it without knocking it off the table, you might find that there are only a few ways you can do it, or maybe there are endless possibilities.
The density theorem is basically telling us that the Kleinians behave in a similar way on the surface of S. They can only move around in certain ways without leaving the surface, and there are either only a finite number of ways they can move or an infinite number of ways they can move.
So why is this important? Well, the density theorem helps us understand how the Kleinians move around and interact with each other in Kleinian space. It also has some really cool applications in mathematics and physics, but that's a story for another day.
So there you have it – the density theorem for Kleinian groups explained like you're five years old!