Tooth coating?!

So, I swung by my dentist's office the other day
and they applied this coating to the inside of my upper front teeth and the
outside of both my top and bottom front teeth.
Apparently, it’s some kind of protective layer designed to slow down tooth wear and tear.
The thing is, it has this... let's call it "unpleasant" aftertaste.
Does anyone actually know what this stuff is, and more importantly, is there any reason to worry about side effects?

Fluoride? If the tube has a picture of a kid on it and it’s clear, you’ve found the good stuff. Too bad they don't make it anymore :-( honestly though, it was a total game-changer for keeping teeth strong!

I’m thinking it came from one of those small bottles. There were quite a few of those little ones lying around—brown ones, green ones... you know how it goes.