Click-through rate (CTR) is a metric that measures the number of times users click on a link compared to the number of times users saw the link (also called an impression). It's expressed as a percentage and can be calculated with the following formula:
CTR = Clicks / Impressions
For example, if users saw your link 1,000 times and 35 people clicked on it, your CTR would be 3.5% (35 / 1,000 = .035).
CTR is used to measure the effectiveness of PPC and other types of online marketing / advertising. It's also a factor in determining organic traffic on a SERP, since both ranking and CTR affect the amount of organic traffic your site receives.