A Clickthrough Rate (CTR) is the amount of people who clicked on the add and arrived at the destination site in a percentage.
If you do not know what a Clickthrough is, please have a read of that post first.
Now a Clickthrough rate only can give an answer regarding those that physically clicked on the ad. It does not include the people who saw the ad and did not click on it yet went to the site later.
This can be a great way of seeing how effective your ad is and to see whether it is bringing in your desired target.