In your statistics, you will find a column labeled Requests and another column labeled Impressions. This article explains what each of these metrics mean.
When a visitor of your site opens a web page that contains your zone invocation code, the browser will download the page and thus also receive these invocation codes. The browser will execute the invocation codes, and for each of the zones, an ad request occurs. Each ad request is received by the ad server, and counted. The ad server then processes the ad request, and selects an ad based on the delivery algorithm inside the ad server software. The resulting ad code is returned to the browser.
The web browser renders the ad code, which means it is displayed so that the visitor can view the ad. Along with the ad code, the ad server has also returned an invisible pixel, called an impression log or impression counter. The fact that this invisible pixel is displayed sends a signal back to the ad server. The ad server then counts an ad impression.