Stereotypic movement disorder is when someone does the same movements over and over again without wanting to. Like when you play with a toy and keep pushing the same button again and again even when it doesn't do anything new.
Some people with this disorder might sway back and forth or flail their arms around a lot. They might do this because they feel happy or excited, or because they feel anxious or stressed.
This is a disorder because these movements can disrupt daily life and activities like going to school, playing with others, or getting important tasks done. But it's important to remember that people with this disorder can still lead happy and fulfilling lives with the proper support and care.