Ok this isn't going to be really fair because the question asked and answered here are more complicated than "what is a skin". But I have yet to see TeamUltimaker answer this question and since this was about skins and got convoluted just like their answers to "what is a skin", I thought I would bring it up here.
Why is it the public can express these concepts better than the people who create them?
See this link
https://www.reddit.com/r/3Dprinting/comments/afimpy/cura_what_is_top_surface_skin_layers_and_why_does/
This may be why the pop up help balloons aren't very good either. Setting: "Skin Edge Support Thickness". Ballon: "Thickness of the infill that supports the skin edges"
Remember when the teacher said you couldn't use the word you were to give a definition for in the definition itself? Ya, that!