If you've visited any forums where people discuss AdSense, you'll see all kinds of abbreviations and hear all kinds of terms that you might not understand. Also, when you log in to your AdSense account for the first time and look at the reports, you'll see a number of abbreviations as well. Here's a list of the most common ones, and a plain-english description of what they mean:

Page Impressions

When somebody goes your website and loads a page in their web Browser, Google counts that as one "page Impression". That means one pair of eays has seen one page of your site. If that same person browses around and looks at 20 pages of your site, Google counts that as 20 page impressions. So basically a page inpression is just how may of your pages were looked at by people.