Okay kiddo, so sometimes people do bad things. And when people do really really bad things, like hurting others or even killing them, the law says they should be punished for it.
But sometimes, people do those bad things because there's something wrong with their brain or their thinking. Like maybe their brain is sick or they don't understand what's right and wrong.
When that happens, the law says they're "not criminally responsible" for what they did. That means they won't be punished like a regular criminal.
Instead, they might have to go to a hospital or get special help so they can learn how to be safe and not hurt anyone anymore. Does that make sense, kiddo?