Click-Through Rate (CTR) is a metric used to measure the success of an online advertising campaign, an email marketing campaign, or organic search listings. It is calculated by dividing the total number of clicks your link receives by the total number of impressions (times it was seen), expressed as a percentage.
For example, if your Google Ad is seen 100 times and clicked 5 times, your CTR is 5%. A high CTR is a strong indicator that your audience finds your ad, email subject line, or SEO meta title highly relevant and compelling. Improving your CTR lowers your advertising costs (in PPC) and can improve your organic search rankings.