A quick art question!

[☀SunBoy☀]

This isn’t a super urgent question but can anybody explain red lining to me? I’ve seen people requesting others to redline their line art in the editable okeaki before and I’m just not sure what it is,,
I’ve watched somebody redline a background but they didn’t explain what was going on, so I was wondering how it works/what it’s for..

[Keir;]

It's used to correct and point out flaws, usually in anatomy.