Concept mastery
Concept mastery is how well you actually know one single idea, measured and tracked over time instead of guessed. It rises when you get that idea right and slips back down when you leave it alone, so your study plan can always see what still needs work.
Mastery is the honest version of "I think I know this." A score per idea tells you the truth: this one is solid, that one is shaky, this other one you got right once a week ago and have probably half-forgotten by now. That last part matters. Your brain forgets things on a curve, so a concept you nailed on Monday is not the same strength by Friday.
It works by watching how you do, concept by concept. Get a question right and that idea's score goes up. Miss it, and the score drops. And even when you do nothing, the score is allowed to fade with time, the way real memory fades. So the weak and the going-stale ideas float to the top, and those are the ones worth drilling next.
The common mistake is studying what feels easy. Re-reading the chapter you already get feels productive, but it does almost nothing. Mastery flips that. It points you straight at the ideas you keep missing or are about to forget, which is exactly where your time pays off.
Priya is studying for a biology exam with 40 concepts in her course. Most are green, but "the Krebs cycle" keeps coming up red because she misses it on every quiz. Her practice keeps handing her Krebs questions until that score finally climbs, instead of wasting her on the stuff she already knows.