Click Through Rate is a ratio showing how often people who see your ad end up clicking it. CTR can be used to gauge how well your keywords and ads are performing.
CTR is the number of clicks that your ad receives divided by the number of times your ad is shown expressed as a percentage (clicks ÷ impressions = CTR).
For example, if you had 5 clicks and 1000 impressions, then your CTR would be 0.5%. Here's how it's calculated: