Okay kiddo, imagine you are playing a game where you have to make choices. Now, what if you were sick or hurt or taking medicine that made it hard to think clearly? You might not be able to make good choices like you normally would.
That's what "diminished capacity" means in United States law. It means that someone's ability to think clearly, understand what's going on, and make good decisions is not working like it should. People can have diminished capacity because of things like mental illness, intellectual disability, or brain injuries.
Now, when someone with diminished capacity does something that breaks the law, like stealing or hurting someone, the law has to decide what to do about it. In some cases, the law says that the person with diminished capacity didn't really understand what they were doing wrong, so they can't be held responsible for their actions. This is called a "diminished capacity defense." It's kind of like saying that the person with diminished capacity didn't really cheat in the game because they were sick or hurt.
However, using a diminished capacity defense is not always easy, and sometimes it doesn't work. The law still expects people to follow the rules and be responsible for their actions as much as possible. But if someone really can't understand what they're doing or don't have control over their actions, the law may show some mercy and give them a break.