system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How does a car scratch remover work?
gpt
It's a very fine abrasive material, held in a liquid suspension.  Think of sandpaper...and the rough abrasive on it.  Now think of *extremely* fine abrasive...and mix it into a liquid.   The liquid lubricates the area as you polish it...it keeps the cutting action of the abrasive smooth and controlled.  

A scratch basically a rough groove cut into the surface of the paint.  The outer layer of the paint is normally very smooth, and so reflects light well.  Scratch it, and you damage this surface...and you will see a whitish line.  Scratch remover basically allows you to polish the scratch down so that it's not so deep, and make the surface reflective again.  Modern car finishes have a clear coat layer over the colour layer.  If you are lucky, the scratch only is in the clear coat...where you can smooth and level this surface with the remover.    Put another way, you aren't  really filling the scratch in, so much as removing the material around it.  Some scratch removers have pigments in them...to help colour the damaged area.