Okay, so you know how sometimes we have rules and instructions to follow to keep us safe and things running smoothly? Like how we have to wear a helmet when we ride our bike or how we're not supposed to touch the stove because it's hot and can burn us?
Well, sometimes people don't follow those rules and do things they're not supposed to do. That's called misusing something.
A misuse case is when we think about all the different ways someone could misuse something, like a tool or an app or a computer program. We want to make sure that the thing we made works properly and won't cause anyone any harm if it's misused.
So, we brainstorm all the different ways someone could misuse the thing and come up with ways to prevent that from happening. It's kind of like thinking ahead and being prepared, so everything works the way it's supposed to and keeps us safe.