If you can’t explain something in simple terms, you don’t understand it

Sadly it's 2017 and the popularity of TEDTalks make the laymen think otherwise.

Opposite example: Simplify how walking works, and make sure to include the critical systems such as major muscle groups, stabilizers, vision, inner ear, thigh/knee/pelvis/hip construction, the curved spine and its connection to the head, and blood pressure flow/regulation.

The question, of course, is how well this experiment has succeeded. My own point of view — which, however, does not seem to be shared by most of the people who worked with the students — is pessimistic. I don’t think I did very well by the students. When I look at the way the majority of the students handled the problems on the examinations, I think the system is a failure. Of course, my friends point out to me that there were one or two dozen students who — very surprisingly — understood almost everything in all of the lectures, and who were quite active in working with the material and worrying about the many points in an excited and interested way. These people have now, I believe, a first rate background in physics — and they are, after all, the ones I was trying to get at. But then, “The power of instruction is seldom of much efficacy except in those happy dispositions where it is almost superfluous. ” (Gibbon)

Richard P. Feynman, 1963

http://feynmanlectures.caltech.edu/I_91.html

Note that by his own account, most of his students did not do well. James Gleick's biography of Feynman, Genius, has a longer discussion of the disappointing results of his lectures to undergraduates at Caltech, many of whom reportedly stopped attending the lectures as they were not getting anything useful out of them.

That Feynman in fact had difficulty explaining freshman physics to the highly qualified students at Caltech surely does not indicate he did not understand freshman physics.

Some topics are simply very complex. It is not clear that they can always be conveyed in simple terms. In some cases, a "big picture" explanation may be possible but the details remain complicated. In some cases, a hand-waving analogy to some everyday phenomenon may create the illusion of understanding but be misleading or wrong.

To give a specific modern example, a state of the art video codec such as H.264 is extremely complex, built of many complicated components and sub-algorithms. While it may be possible to explain the big picture in relatively simple terms, the detailed implementation and operation is not simple. The inability of someone who creates or implements a video codec to explain it in simple terms to a layman is not an indication that they do not understand it.

Yes, but knowing which information you can lose is why it is said that you don't understand it until you can explain it. Explaining it in simple terms forces you to over-simplify, and then you have to pick which information is important and which isn't...and doing that requires a deep understanding of the subject (and your audience).