The act of simply putting a thought into words makes it immediately obvious to you if you really understand it or not, and if not, where your blind spots are.
If the thing you’re concerned with is an unresolved problem or a question, simply articulating the problem or the question can make the solution or the answer obvious. Just going through the process of putting it into words, one way or another, and being sure you’ve settled on the most concise and accurate description of it you can muster, will often make so many things that were hazy obvious, and can reveal to you areas of haziness in your own thinking that you may not have been aware of.
Rubber duck debugging is one result of this. Rubber duck debugging is based on the observation that by the time you’ve explained your bug to someone, often times you figure out your next steps before they even respond. Just articulating your problem in the form of a question asked to someone else will sometimes reveal the answer to you.