There’s a quote that’s often attributed to Albert Einstein which goes:

“If you can’t explain it simply, you don’t understand it well enough.”

Whether or not Einstein himself actually said this (it’s never been properly sourced, so it’s likely he didn’t), it’s still an insightful observation. It’s also one that yields a pretty powerful study tip when reversed:

If you want to understand something well, try to explain it simply.

By attempting to explain a concept in simple terms, you’ll quickly see where you have a good understanding of that concept. You’ll also be able to instantly pinpoint your problem areas, because they’ll be the areas where you either get stuck or where you end up resorting to using complex language and terminology.

This is the idea behind the Feynman Technique.