You access the Ad Manager using the Marketing Menu in your store.
The Quick Stats show you some overview statistics on the performance of your ads. These are aggregate numbers for all of your ad spots. Data can be affected by site redesigns that include more Ad Spots, and by the nature of the Ads that you are running.
Impressions are the number of times your ads have appeared on your web pages.
Clicks show the number of times that a visitor has clicked on an ad image.
CTR is the Click Through Ratio for all of your ads. This is the most important number to monitor across all of your ads.
The Data Controller at the bottom of the screen allows you to generate reports on your Ad activity. The reports are loaded onto the page below the data controller as shown above.