ELI5: Explain Like I'm 5

Convex regular 4-polytope

A convex regular 4-polytope is like a special kind of shape that has exactly four dimensions. This shape has a special property, where every face is a regular polygon and all the edges and angles are the same.

It's kind of like a ball that has only one type of flat surface, like a bunch of equally-sized and shaped tiles covering it up. But instead of being a ball, it has four dimensions, meaning there are four directions you can move in to explore its shape.

Because all the shapes are perfectly regular, this 4-polytope is very symmetrical and looks the same from every angle. It's a very special and unique shape that mathematicians find interesting and fun to study.