Stationary" cavities?

Even Uncle Sam doesn't seem to recognize this term; I couldn't find anything about it in either American or English sources.
Apparently, we're talking about a type of cavity that supposedly doesn't need to be "touched" because it's "localized and won't spread to other teeth." At least, that's what my dentist at the local clinic told me.
Before I jump to any conclusions, does this actually make sense? If it does, could someone weigh in from a professional standpoint?

Personally, I don't see any reason not to just clear out the decay and get a filling done while I can. My goal is to keep my teeth healthy for as long as possible. Though, maybe that's a luxury not everyone with basic insurance can afford $3.25...

Thanks!

I’ve encountered that term "suspect tooth" before; my dentist tends to prefer calling it a questionable tooth rather.

Let’s say you had some initial decay in a tooth fissure, but then your hygiene improved and the cavity stalled out. If it's very shallow and just looks like a small brown spot now, it might stay that way indefinitely, or it could potentially reactivate down the road. It isn't an emergency to have it cleaned immediately, but you should definitely keep an eye on it over time.

Gerald Smith76 said:Let's say you had some initial decay in a fissure, but then your hygiene improved and the cavity stabilized. Now it's just a shallow, brown spot... that can stay like that forever, or it could suddenly flare up again at any moment. It's not an emergency fix, but definitely something to keep an eye on over time.

Thanks for the insight.
That said, if we're looking at fillings that are already ten years old and a dentist can feel some breakdown using a dental probe, would it be smarter to just grind it down and check how deep the decay actually goes? What are the odds that the tooth keeps rotting (even without pain) if I just leave it alone for another year or two?
Or to put it another way, how does a dentist tell during a checkup that a cavity has reached the point where it absolutely needs to be cleaned out?

If a filling has a crack in it, it needs to be replaced. After ten years, it’s likely reached the end of its lifespan anyway. You can't truly see the extent of the damage until it's removed; it could just be some minor surface decay at the top, or it could be a deep cavity underneath.

I had a feeling...
Thanks again!