Completion rate

Completion rate answers one question: of everyone who pressed play, how many were still there at the finish. Short-form platforms report it directly, because their videos are short enough that finishing is the normal outcome. YouTube Analytics gives you audience retention instead, so the figure comes from where the curve sits at the end of the video.

Length moves it more than quality does. That is why comparing completion across videos of different lengths tells you very little. Compare one video against others of similar length on your own channel and the number starts to mean something.