I've read (and tend to belive, based on my limited experiences) that the "what" you're "correcting" is the edges of scratches and marring. While many times, you can probably lower the surrounding clear to the valley-point of very superficial marring, lots of the times you're rounding the edges surrounding this valley. That causes the defect to "go away" - meaning that the light doesn't reflect off the sharp edges anymore, and you don't see it anymore. Essentially, as Bryan mentioned above, you're trying to get the topmost surface of the paint as flat/level as you possibly can so that light (i.e. reflections) are not distorted by the unevenness of the surface defects.
In a nutshell, you don't actually "remove" marring, you remove surrounding "good" paint to get as close/uniform to the defect bottom as possible.