CTR stands for Click through Rate. It is calculated by dividing the number of times a link appears on a search engine result page (impression) by the number of times it is clicked by users. For example, if you have 10 clicks and 100 impressions, your CTR would be 10%.
The higher the clicks, the higher will be the CTR. A high click-through rate is essential for a successful PPC, i.e., the success of a PPC depends on the CTR. Thus, it is an important metric in PPC ads which helps you gauge the results and tells how effective your campaigns are.