Inter-universal Teichmüller theory is a really big and complicated idea in math that is hard to explain, but let's imagine you have your favorite toy, and you have another toy that looks exactly like it, but a bit different in color or shape.
Mathematicians study things like this all the time, and they call them "moduli spaces." What they do is, they take all the possible shapes and colors that your toys could have, and they put them into a big space called the "Teichmüller space," which is like a big room filled with all the possible toys.
But here is where it gets more complicated: Imagine you have two totally different rooms, they don't look alike at all, but they have similar toys. If you want to compare the toys in one room with the ones in the second room, it is not an easy task because the rooms are different.
Inter-universal Teichmüller theory is like having a magic tool that can help you compare these two rooms even though they look different. Imagine this magic tool like a smartphone that can take pictures instantly and send them to each other.
So, mathematicians developed a way to compare different Teichmüller spaces by using a new kind of math that involves really crazy things like symmetry and topology. They named this idea as Inter-universal Teichmüller theory.
It is a really big deal in math, and it can help us understand better how different spaces are connected to each other, even if they look totally different at first. But it is still a really difficult topic even for grown-up mathematicians to understand completely.