A default skin completely replaces the EA skin, so you can't choose it in the skintone panel.
A non-default skin is something you can choose and doesn't completely replace the EA skin.
Edit: a better worded response from someone on the forums; "Default replaces the ones that are originally in the game, whereas non-default ones are separate ones, you can choose whether you want to use them or not."
thank you so much for letting me know that and did you see my other question on here? noone has answered it yet and i really need some help, so if you could read it and if you know any of the answers to the questions i would really really appreciate it. thank you again for taking the time to answer my question. have a great day!