Stimming is different from person to person, but it is a behavior that is typically repetitive that is used as a means to soothe an unwanted sensation/emotion/state of mind. Sometimes, stimming is entirely unintentional and to force a person to stop stimming can sometimes exacerbate the issue they’re trying to deal with.
Stimming can be things as simple as chewing on a #2 pencil. It could be curling in a ball and rocking back and forth. Some people have vocal stims, like randomly meowing (as I used to do when I was a kid when I felt uncomfortable). There are literally ENDLESS ways people stim. Jumping around, hand flapping, screaming, flailing, running, rocking, bouncing your knee up and down…the list goes on.
Stimming, when I see it spoken about on Tumblr (and this is mostly because of the circles of people that I follow, is associated with Autism. However, stimming is not an Autism-only behavior. There are many neurotypical people who stim without thinking, there are many neurodivergent/non-autistic people who stim.
Stimming just seems to be frowned on a lot by neurotypical people when it has to do with Autistic people (or really anyone who does it in a way that cannot be passed as ‘socially acceptable’) does it. They’re forced into therapies that try to recondition the behavior and are shamed for it.
When really, it should never be treated as a bad thing. Even in the instance of hurting oneself as a stim, rather than shaming it, they should simply be guiding them toward a safer stim that can be substituted for it (instead of biting ones self, bite an inanimate object)
I hope this helps! (I may have missed stuff, someone feel free to reblog and correct me if needed)