For the non-autistic person this term may be unknown. Stimming is short for Self-Stiimulating Behavior. The primary reason an autistic stims is to cope with the environnment in which they live, much like a neurotypical person uses a stress ball to cope with stress. It produces the same effect as the stress ball because for some reason the movement of the body and the stimulation of the nerves distracts the brain from the dealing with emotions such as anxiety or joy or even sadness.
I made this video to show how I stim.