Hey kiddo, sometimes people have a really hard time thinking and understanding things, even things that seem easy for other people to understand. This is called severe mental impairment. Imagine that your brain is like a computer and when it is "broken", it can't do all the things that it is supposed to do.
People with severe mental impairment may have trouble talking, learning new things like math or reading, or even understanding what is happening around them. It's like they are seeing the world through blurry glasses or hearing things through really muffled ears.
But just like how we can get help when our body is sick or broken, people with severe mental impairment can get help too. They may go to special schools or have special doctors who can help them with their unique needs. It's important to remember that just because someone thinks differently than we do, it doesn't mean they are any less important or valuable. They just need a little extra help and love.