Click-through rate
Also called CTR · impressions click-through rate
The share of YouTube impressions that turned into views: how often people who saw the thumbnail chose to click it.
In more detail
YouTube reports impressions click-through rate as clicks divided by impressions on the platform's own surfaces. It measures the title and thumbnail together, since that pairing is all a viewer sees before deciding. It is not comparable between channels or even between videos on one channel, because the rate depends heavily on where the impressions came from.
Example
A video shown mainly to existing subscribers on the home feed will post a higher rate than the same video shown to strangers in browse, without being better packaged. Comparing a video to your own recent uploads in similar placements is informative; comparing it to a number someone quoted as a benchmark generally is not.
Why it matters
Click-through rate and retention answer different questions and are easy to confuse. A high rate with poor retention means the packaging promised something the video did not deliver. That is worse than a low rate, because it teaches YouTube to stop showing the video. Reading them together is the only way either number means anything.