What is this ridge inside my toilet flange? Do I need to replace it?
It looks like you have a standard toilet flange, but it's attached to a 3" pipe instead of a 4" (that's what the 4x3 on top means). It could also be a "push fit" flange that you push into the drain pipe to replace an older flange. I admit this looks weird, because I would expect the inside lip to be sloped downward so sewage can't sit on top.

The flange itself is low, but appears to be in good shape. I wouldn't replace it just because of the lip. It looks like that's standard to all the Charlotte 4x3 flanges I've seen.