How do you know whether you have really understood something?

"The physicist Ernest Rutherford once said, in essence, that if you can't explain a result in simple, nontechnical terms, then you don't really understand it. He wasn't saying that this means your result is wrong; rather, he was saying that it means you do not fully understand its origin, meaning, or implications."

Brian Greene, "The elegant universe" (p. 203).

This rejoins this famous French quotation:

"Ce que l'on conçoit bien s'énonce clairement,
Et les mots pour le dire arrivent aisément."

Nicolas Boileau, "L'Art poétique".