Clickthrough rate (CTR) is the number of clicks your ad receives divided by the number of times your ad is shown (impressions) via Google search only. Your ad and keyword each have their own CTRs, unique to your own campaign performance.
Keyword CTR is a strong indicator of the relevance of your keyword to the user and the overall success of the keyword. CTR is also used to determine your keyword’s Quality Score. A low CTR may point to poor keyword performance, indicating a need for ad or keyword optimization.