If you can’t explain it simply, you don’t understand it well enough.

The saying links genuine understanding with the ability to communicate clearly. It suggests that mastery is not merely possessing technical knowledge, but being able to distill an idea to its essential structure without hiding behind jargon. In science and teaching, this functions as a practical test: if an explanation collapses unless it remains complicated, the explainer may be relying on memorized procedures rather than grasping underlying principles. The quote also implies an ethical dimension—clarity is a service to others—while acknowledging that simplicity here means conceptual clarity, not oversimplification that distorts the truth.