It would depend on their ability to understand, I believe, as some NTs are more progressive than others. Mine are usually active when in places with loads of sensory stimuli, such as thoroughfares with lots of oncoming traffic and people milling about. I zone out, fidgeting with the strap of my bag almost constantly, looking down at my watch and pretending to be concerned by the time. It's all an act, of course, but it's all about maintaining the illusion that I'm like them. Other times, I don't care who sees me stimming, and I'll rock gently back and forth in my seat, but I doubt that anyone with only basic-level knowledge of Autism would assume that I was stimming.
You could try telling them that it's a physical means of processing aberrant or errant thoughts or thought patterns though, if that's not too technical for them to comprehend. Or, you might want to use analogies of some kind if you are able to understand and use them yourself, as NTs respond very well to analogies and mental images that amount to the same thing. My favourites are the washing machine or tornado metaphors, that my thoughts are sometimes like either of the two, using debris or laundry as stand-ins for racing thoughts. Every now and again, my head goes through "spin-cycles", cue a peak in anxiety levels and risk of panic attack, necessitating the stimming. It's about bringing yourself back from being in "spin-cycle" all the time so that you can function in day-to-day life.
Really, however, that's why I stim; it might be different for you. I also stim for comfort or when I'm in a good (or bad) mood, but that is much harder to explain to NTs, unless you relate it to de-stressing or something. Hope that helps