Okay kiddo, interchangeability in computer science means that you can use different things that do the same job in the same way. It's like how you can use different kinds of toys to play the same game. For example, you can use a ball or a frisbee to play catch, and they both work equally well.
In computer science, interchangeability happens when you can swap out one piece of code or software for another and it won't make a difference to how the program works. This makes it easier to update or maintain a software program because you can switch out parts without having to change everything else.
It's like having a puzzle where all the pieces fit together no matter which way you flip them. That way you can choose different pieces to finish the puzzle and still end up with the same picture.
Overall, interchangeability is a cool trick that makes it easier to work with computer software and allows us to have lots of options for different parts that all do the same thing.