CTR stands for Click-through-rate.
Uttered in a percentage, a click-through-rate refers to the number of clicks related to the number of impressions. For example, let’s say that 2 people clicked on your banner out of 100 impressions — this would mean that your banner has a CTR of 2%.
Knowing something's CTR is a way of measuring the success of a campaign.
For additional terminology and key performance indicators for measuring success, please see: