Explaining shows what you know
Reading something and nodding along can feel like understanding. The real test is whether you can explain it yourself. When you try, you often find the explanation falls apart at a certain step. That moment is very useful, because it shows you exactly what you have not learned yet.
This is called self-explanation when you do it for yourself, and it works for almost any subject. As you read or solve a problem, stop and ask: what does this step mean, and why does it follow from the last one?
Use plain words
Try to explain the idea as if to a curious twelve-year-old. Skip the jargon. If you catch yourself using a fancy term, stop and say what it means.
Plain words matter because big terms can hide confusion. You can say "the market reached equilibrium" without knowing what is going on. Saying "the price settled where the number of people wanting to buy matched the number of things for sale" is harder, and you can only do it if you get it.
Examples help too. If you can explain an idea and then give an example of it, you are on solid ground.
Teach someone, or something
Explaining to another person is a strong version of this. A friend, partner, or coworker can ask questions you did not think of. Even planning to teach can make you study more carefully.
No one around? Talk to an object. Programmers have a habit of explaining a stubborn bug, line by line, to a rubber duck on their desk. Saying each step out loud often reveals the mistake. A pet, a plant, or a voice memo works just as well.
Go back and fill the gaps
The explaining is only half the job. When you get stuck or feel unsure, mark that spot. Go back to the book, your notes, or a teacher, and work on just that piece. Then explain the whole thing again.
One caution: a smooth explanation can still be wrong. Feeling fluent is not the same as being right, so check your version against a trusted source.
Try it
Pick one idea you learned recently. Set a timer for three minutes and explain it out loud in plain words, as if to a friend. Write down every spot where you paused or guessed, then look up just those parts.