Could you explain what is stereotypic movement disorder?

Stereotypic movement: Stereotypic movement disorder is a condition in which a person engages in repetitive, often rhythmic, but purposeless movements. Sometimes these movements can result in physical injury, as they may include head banging, biting nails or other body parts, self-hitting, skin-picking, mouthing of objects, etc. It most often affects kids with neurological disorders, and can increase with stress, etc.