Explanation
Short-form platforms decide who to show a post to by testing it on a small audience and reading the response, and retention is the loudest signal in that response. It is a more honest measure than likes because it is passive: a viewer does not have to decide to give it to you.
The shape of the curve matters more than the average. A cliff in the first two seconds is a hook problem. A steady slide is a pacing problem. A drop at a specific moment usually means you said something that gave people permission to leave, which is often a summary of what is coming rather than the thing itself.
Loops and rewatches can push retention above a hundred percent on very short videos, which is why the shortest formats behave strangely and why a longer video needs a real reason to be longer.
The common mistake
Chasing average watch time by making videos shorter and shorter. Retention rewards satisfaction, not brevity, and a video that ends before it delivers gets rewatched by nobody.