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

Summary

This quote suggests that true understanding of a concept or idea can be measured by one's ability to explain it in a simple and concise manner. It implies that deep understanding is shown by being able to break down complex topics into clear and understandable explanations. If someone struggles to simplify a concept, it may indicate that they lack a comprehensive understanding of it. Simplifying complex ideas is a valuable skill that demonstrates true mastery of a subject.