Stimming is short for self-stimulation. It can be visual (fingers in your face, strobe lights, blinking rapidly, pressing your eyeball), vocal (coughing or clearing your throat, screaming, whispering), oral (licking, sucking), tactile (touching stuff, rubbing stuff), kinetic (rocking, running, jumping), auditory (scripting, making sounds, playing something over and over), olfactory (sniffing everything), etc. etc. Anything to stimulate your senses.
We stim all the time, but like the PP said, it's a problem when it's either not socially acceptable or interferes with your daily life. This applies to everyone, not just people with ASD. It's also not exclusive to ASD in the realm of disabilities/disorders - many people with various disabilities stim. I crack my knuckles and play with my hair. My DH chomps his gum ridiculously loud, I could pop my gum until everyone around me goes crazy. But chewing gum is socially acceptable. Chewing your shirt collar is not.
There are various ways to deal with stimming. You can try to extinguish it (get rid of it) which will require a replacement behaviour for it (e.g. licking a sucker instead of your hand), or you can limit it to certain times/places. It's a soothing thing for people, so as long as it's not harming anyone, it can be fine. Only you can judge that though. Some people also can't come 'out' of a stim phase if you let them get into it and just wind up tighter and tighter. Others just need to get it out and then can get on with their day.
That's all I've got.