Above the fold is a term borrowed from print media (referring to the fold of the newspaper) and refers to the placement of an ad on a page. An ad that is viewable as soon as the web page populates is considered "above" the fold. 

An ad requiring a user to scroll down (or sideways) to see it is considered "below the fold."

Since screen resolution can affect what is immediately viewable, it's good to know whether the website's audience tends to set their resolution at 640 by 480 pixels or at 800 by 600 (or higher).