Stimming

Can you explain what stimming is in a simple way?

I don't know if I stim

I know I make a noise with my breathing through my nose, but I don't know why I do it. 

When I'm in bed I like to rub my foot on the sheets. I know this makes me relax and calms me. Is that a stim?

I used to suck my thumb and touch a cold pillow with the back of my hand when I was a child. I know this relaxed me when I was younger

What else could me a stim?