This calculator helps you estimate how much you will earn given certain traffic figures or how much traffic you need in order to reach your revenue goals. For example, I know I need my website to generate $5000 of ad revenue. I also know that on average I'm earning $4 per 1000 page impressions (from all ads on the site, AdSense provides this figure). This tells me that I need 1.25 million views to reach my goal. If an average user sees 3.2 pages, then I need a little over 390000 visits. Please keep in mind that we're not distinguishing between particular banners on the page - all advertisements are treated as one. So it's revenue per page impression, not revenue per banner impression.
If you're on the other side of the deal (you buy traffic instead of selling it), you may also be interested in a CPM calculator for a basic cost per thousand impressions calculation or a online marketing funnel one to simulate the whole journey from impression through CTR, clicks, leads all the way up to sales.