Okay kiddo, so mental illness in fiction means that sometimes in books, movies, or TV shows, the characters might have problems with their thoughts, feelings, or behavior. Just like when some people get physically sick, some people can also get mentally sick.
This can be shown in fiction in many different ways. Sometimes, the character might seem really sad and cry a lot, or they might feel really worried and scared all the time. Sometimes they might even see or hear things that aren't really there, or have trouble understanding the world around them.
It's important to remember that mental illness is a real thing that affects real people, just like physical illnesses. Sometimes, showing these problems in fiction can help people understand and relate to what others might be going through. But it's also important to be kind to people with mental illness and to understand that it's not their fault that they have these problems. Does that make sense, kiddo?