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 one of the three Quality Score components, and a real-time estimate of it feeds Ad Rank in every auction, 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.
Common questions
What is a good CTR?
It depends on the channel and intent. Search CTRs run far higher than Display or paid social because the searcher already has intent. Compare like with like, and watch the trend more than the absolute number.
Does CTR affect cost?
On Google, yes. Expected CTR is one of the quality signals in Ad Rank (and one of the three Quality Score components), so a stronger CTR can lower your cost per click for the same position.