ELI5: Explain Like I'm 5

Cut locus (Riemannian manifold)

Imagine you are a tiny ant walking on a bumpy surface that stretches out infinitely in all directions. You can walk around and explore this surface, but sometimes you reach a spot where you can't go any further without encountering a barrier, like a wall or a cliff.

If you were to map out all the spots where you can't go any further, you would end up with a shape that looks like a bowl with lots of little dimples and bumps. This shape is called the cut locus.

Now, let's imagine that instead of an ant, you are a traveler trying to get from one point on the surface to another. If you set out in a straight line, you might find that you run into one of these barriers from the cut locus, and you have to change direction to go around it. This can make your journey longer or more difficult.

The cut locus is important in Riemannian geometry, which is a type of geometry that deals with curved surfaces. By understanding where the cut locus is on a Riemannian manifold (a curved surface), mathematicians can better understand the geometry of that surface and how objects move on it.