The Click-Through-Rate or CTR is Google’s prediction of how often an Ad will be clicked on when it is shown. To calculate this prediction Google uses the enormous amount of data that it has on user behaviour.
To be more specific, the CTR refers to the number of clicks that your ad receives divided by the number of times your ad is shown. Thus: Clicks ÷ Impressions = CTR. For example, if you had 10 clicks and 100 impressions, then your CTR would be 10%.
The Google Ads Help page explains: “A high CTR is a good indication that users find your ads helpful and relevant. CTR also contributes to your keyword’s expected CTR, which is a component of Ad Rank. Note that a good CTR is relative to what you’re advertising and on which networks.”