What are Ad Impressions ?
So impressions are basically how many times your ad loads up on a page. So lets say you have a banner on one of the sites; and it says you pay $3 Per 1K Imps.
Basically it is saying that; every 1000 times your banner loads up on that page you are advertising on; you will pay $3. Now you never know if people actually saw your ad or not; again it is not based on who sees it but on the number of times it loads up on a page.