What is this insect

They don't hurt humans or pets. They'll also tend to hide, given the opportunity.

They do like to eat paper, however. One silverfish isn't a big deal, but if you see them often it's worth looking into deterrents or pest treatment. When there are lots, you'll start to notice holes in notes, books, etc — most particularly (in my experience), loose pieces of paper left undisturbed for a length of time.

They are harmless per se, however if they appear suddenly they might be an indication of a water leak or humidity to which they're drawn to. They like to settle on starchy materials: books, wallpaper that sort of stuff.

Not harmful, mostly an annoyance.

It's the gray or long-tailed silverfish: https://en.wikipedia.org/wiki/Ctenolepisma_longicaudatum

They can be somewhat of a pest if they are in your house (unlike regular silverfish, who are relatively benign) and if you have books, or other papermaterial stored.

They are indeed harmless, but they can multiply a lot and ruin furniture if left undisturbed. My mom spent this winter dealing with an infestation in her house.

If you keep seeing them, you should take action before they can get out of hand

They eat mold afaik. So if you see them, you know there's a source of humidity/mold you ought to check out.

Eating books and paperwork.

They eat bugs and stuff. Not spiders though, that's a myth (unless they're already dead maybe).