Click-through rate (CTR) is the share of people who clicked after seeing your ad, calculated as clicks divided by impressions. It is the clearest signal of whether your ad and audience match, and on Search it feeds Quality Score.
What CTR really measures
CTR is clicks ÷ impressions. A healthy CTR says the right people are seeing a relevant message; a falling CTR on a running campaign is a classic sign of creative fatigue or a saturating audience.
Where it feeds back into the system
On Google, expected CTR is a direct input into Quality Score and therefore your Ad Rank, so a better CTR lowers what you pay per click. On Meta it shows up in the relevance and engagement signals that shape delivery. A high CTR on the wrong audience still is not the goal, so read it next to conversion rate.