In "physics land" Richard Feynman is famous for his explanatory power, e.g. We tried to take advantage of Richard's talent for clarity by getting him to critique the technical presentations that we made in our product introductions. Before the commercial announcement of the Connection Machine CM-1 and all of our future products, Richard would give a sentence-by-sentence critique of the planned presentation. "Don't say `reflected acoustic wave.' Say [echo]." Or, "Forget all that `local minima' stuff. Just say there's a bubble caught in the crystal and you have to shake it out." Nothing made him angrier than making something simple sound complicated.
Additionally, I've found that writing out the problem or solution in plain English sometimes helps me make the leap from conceptualizing to actually understanding.
I enjoy also the Alan Perlis line, "You think you know when you learn, are more sure when you can write, even more when you can teach, but certain when you can program."
I have been endeavoring to get in the habit of writing about things that I learn. My main obstacle is the feeling that, even if I can write and explain the topic well, other people can too and already have; so why bother writing about it? But that misses the point -- or even two points.
The writing helps me; the fact that someone else wrote is irrelevant. Further, my writing, even if the subject matter is hardly original, might in fact help someone else: not everyone favors the same calculus textbook, or the same Python programming manual, or the same history of World War II. People enjoy different writing styles, different perspectives, even different fonts and page layouts. (I've elected against buying at least one book because I found the page layout excessively annoying to look at!)
Writing about a topic is a good test of whether you can explain it simply.