Understanding is one thing, explaining in layman or easy to undertand terms is another, and sometimes it takes a lot of effort.
You guys would love Richard Feynman.
That collection is a series of lectures from Feynman, and his characteristic clarity, on physics across the breadth of all major topics. It's quite amazing to realize how little you, in general, truly understand even really basic concepts like the conservation laws until you read the writings of a person who did genuinely understand what he was talking about and could share that insight in such a uniquely effective way.
Being able to explain/teach something to others is a very different skill to understanding it and isn't compulsory.