Okay kiddo, so imagine you are playing with a bunch of lego blocks and you build two really cool structures that look kinda similar, but they are not exactly the same. Now, someone tells you that these two structures can actually never be made exactly the same, no matter how hard you try!
This is basically what the Mostow rigidity theorem is all about. It says that if you have two different shapes, called "manifolds", that are connected in a certain way called "isomorphic", then they must have exactly the same shape and no one can change that, just like your lego structures.
But why is this important? Well, imagine you’re a scientist trying to study these manifolds and you learn some things about one of them. With the Mostow rigidity theorem, you can take what you learned about one, and apply it to the other one, because they are identical shape-wise.
The theorem is named after a mathematician called George Mostow, who proved it in the 1950s.