CTR stands for Click through rate. CTR is the percentage of clicks an ad receives. It is calculated by dividing the number of clicks an ad received by the number of times the ad was shown multiplied by 100.
CTR = (Clicks / Impressions) * 100
For example, if an ad has 25 clicks and 1000 impressions the CTR will be (25/1000)*100 = 2.5% which means that 25 percent of users clicked on the ad and got redirected to the app store or play store.