Stimming or self-regulatory behaviour is a common phenomenon in autistic people and a way to process sensory over-stimulation or to regulate emotions.
Compared to neurotypicals (Nts), autistic people have a different way of processing information and are often sensitive to sensory input such as sound or strong light. This can lead to overstimulation and stimming is a way to relax and process that.
Common ways of stimming include repetitive movements such as flapping the hands or rocking back and forth, making sounds or repeating individual words and fidgeting.
Some NTs believe that stimming is bad and should be limited or eliminated, but once you understand why it’s there you also realize that eliminating stimming is a totally wrong way to go. It’s absolutely necessary for an autist to stim to be able to self-regulate and relax and the only thing you can do is to limit the sensory input if a person becomes too stressed.
Stimming can also be done because it’s fun and relaxing, so to aim at eliminating them fully is harmful and very stressful for an autist.