Okay, so imagine you have a big room with a lot of toys in it. You want to divide the room into separate areas for different kids to play, but you have to make sure that every kid has access to the same amount of toys.
Now, let's say you have a rule that you can't move the toys from where they are. So you have to figure out how to divide the room up in a way that works with the toys' positions.
The Hahn-Banach Theorem is kind of like that, but with math. It says that you can divide up a mathematical space (kind of like the room) in a way that works with certain rules (kind of like the rule about not moving the toys).
But the special thing about the Hahn-Banach Theorem is that it says you can do this in a way that includes certain kinds of functions. Functions are kind of like rules that tell you what to do with numbers. And the Hahn-Banach Theorem says that you can divide up the space in a way that works with different kinds of functions, as long as the functions follow certain rules.
So it's a theorem that helps mathematicians divide up spaces in a way that works with different kinds of functions, kind of like how you might divide up a room for different kids to play with different toys.