If you can't explain it simply, you don't understand it well enough.
Some things in life, of course, are just plain difficult to understand. Often enough this is their ineluctable nature (and ours); but if a particular subject seems to offer itself up for mastery, try learning as much as you possibly can about it and see if that doesn't thicken the plot just a bit.
The more you know, the more you don't know.
This second piece of popular wisdom carries a very different message from the first: a depth of understanding sufficient to permit the formation of simple explanations is highly destructive of the willingness to accept them; and thus in the ethical sense, I would say, also to dispense them.
Hence there is a third colloquialism which is an essential corollary to the first:
Just because you can doesn't mean you should.