Ad requests are the number of times that an ad has been called up. For a native ad, this will mean the number of times the ad has been displayed on a website. For a pop-under/direct ad, as these ad types send traffic directly to a landing page, not via an ad, it's the number of times that the ads targeting were matched (potential traffic).
This is simply the number of visitors sent to your target URL. Each visit will be charged according to your bid.
This is the Click Through Rate, a percentage of the number of traffic you are getting from the total potential traffic. A low click through rate may suggest that you need to improve the look of your ad, or increase your bid.
This is the total cost of clicks for the ad, over the time period displayed.