Monotropism is when somebody focuses really hard on one thing and finds it hard to switch their attention to something else, almost like a magnet. Imagine that you really love playing with your toy cars and you can't stop thinking about them, even when somebody is talking to you or you see something else interesting.
People who have monotropism might feel the same way about certain topics or activities. For example, they might be really fascinated by trains or math, and find it hard to pay attention to other things around them. Sometimes this can make it challenging to socialize or do well in school, but it also means that these people can become really good at the things they love.
Think of it like having a favorite toy that you don't want to put down - people with monotropism have a favorite topic or activity that they don't want to stop thinking about!