Banach-Stone theorem is a rule in math that says if you have two boxes (these are sets which can hold other things) and you want to see if they are the same, you don't need to look at everything inside the boxes. Instead, you just need to look at one very special thing called a "function" and see if it works the same in both boxes.
Imagine you have two boxes which are filled with toys. You want to know if the toys inside the boxes are the same or not. To do this, you take out one special toy and look at it. If this toy works the same way in both boxes, then you know that all the toys in both boxes are the same. That is what the Banach-Stone theorem is all about.
Okay, so what exactly is this "special toy" that we need to look at? This "toy" is actually a function. A function is like a machine that takes in one thing and gives back another thing.
For example, imagine a machine that takes in a number and doubles it. So if you give the machine the number 3, it will give you back the number 6. This is called the "doubling function."
Now, imagine you have two boxes which are filled with numbers. To see if the numbers in both boxes are the same, you just need to look at the doubling function and see if it works the same way in both boxes. If it does, then you know that all the numbers in both boxes are the same.
The Banach-Stone theorem is a bit more complicated than this, but this is the basic idea. It says that if you have two boxes (which are actually sets) and you want to see if they are the same, you just need to look at one special function and see if it works the same way in both boxes. If it does, then you know that the sets are the same.