CTR is Click through rate which is calculated as number of clicks on ads divided by the total number of impressions.
It can be simply said the ration of users who click on a specific link to the number of total users who view a page, email, or advertisement.
CTR is an essential factor to see if campaign is successfully worked or not. The higher CTR the more effective the marketing campaign has been at bringing people to a website. It directly affects both your Quality Score and how much you pay every time someone clicks your search ad.