What it is
In online advertising, the click-through rate (CTR) is the percentage of individuals viewing a web page who view and then click on a specific advertisement that appears on that page.

How to use
Click-through rates measure how successful an ad has been in capturing users’ attention. The higher the click-through rate, the more successful the ad has been in generating interest. A high click-through rate can help a website owner support the site through advertising dollars measured in cost-per-click.
It may be utilized with a variety of mediums, such as display advertisements, email advertising, and paid search.
What to consider
A high click-through rate means that lots of users are clicking on an ad, but it does not inform the user about the number of sales the ad ultimately generates by leading to purchases. For this reason, the conversion rate – the percentage of click-throughs that lead to actual sales – may be a more useful metric of an ad campaign’s success.