CTR is a packaging metric. On YouTube it usually refers to thumbnail-and-title clicks; in ads and link-in-bio contexts it measures how many viewers acted on a link. Because it isolates the click decision, it tells you whether your hook is the bottleneck — strong content with a weak thumbnail can have great retention but a low CTR.
What counts as "good" varies wildly by platform and placement, so CTR is most useful compared against your own baseline and tested with experiments. Pairing it with what happens after the click matters too: a high CTR with low retention or conversion can signal clickbait that disappoints, which platforms eventually penalize.
