I would guess is something regards a person that is not very smart (raccoon dog) cannot teach someone who is more intelligent (fox).
As fox are considered/represent smart, usually in a deceiving way, it could also mean that a naive person cannot teach anything to a experienced person.
Just my opinion, though. Someone else might have a better answer/explanation.
I am not sure. I haven't heard this proverb or can't remember it. At least, it is rarely known.
Just in case this was bothering anyone else; I found out that 너구리 is raccoon dog and 미국너구리 (American Raccoon Dog) is how you say raccoon.