A click-through rate (CTR) in Gmail measures how often recipients click on links within an email. It is calculated by dividing the number of clicks on links by the total number of emails delivered, then multiplying by 100 to get a percentage. For instance, if you send an email to 100 people and 5 click on a link, your CTR would be 5%. CTR is significant because it indicates engagement levels with your content. A higher CTR suggests that your email content resonates with recipients and prompts them to take action. In Gmail, tracking CTR can be done by using tracked links, which notify you every time a recipient clicks a link, allowing for better analysis of your email marketing effectiveness.