Clickthrough rate (CTR) is the number of clicks your ad receives divided by the impressions (number of times your ad is shown on Google). In other words, CTR shows what percentage of the time users clicked your ad after seeing it shown on Google.
Your ads and keywords each have their own CTRs. Use CTR to evaluate your ad's relevance to users and the overall success of that keyword. The more your keywords and ads relate to each other and to your organization, the more likely a user is to click your ad after searching on your keyword phrase. For example, a well targeted keyword that shows a similarly targeted ad is more likely to have a higher CTR than a general keyword with non-specific ad text.
A low CTR may point to poor keyword performance, indicating a need for ad or keyword optimization. Therefore, you can use CTR to gauge which ads and keywords aren't performing as well for you and then optimize them.
CTR is also used to determine your keyword's Quality Score. Higher CTR and Quality Score can lead to higher ad position and more exposure within your allowed budget.