What is illusory exactly? Is it the brain misrepresenting its own workings? But those misrepresentations are encoded in the brain in the same way as everything

What is illusory exactly? Is it the brain misrepresenting its own workings? But those misrepresentations are encoded in the brain in the same way as everything else and they do have causal influence on the brain processes. So, those "illusions" are as real as representations of the outside world.