Comment by Nevermark

I would say: You either don't understand your subject, or don't understand your audience, if you can't explain your subject to your audience, at the highest level they can understand, coherently.

The average person can understand anything ... at some level. Being able to match that level is positive evidence (but not proof) of competence.

Duality: Being unable to match that level is positive evidence (but not proof) of incompetence.