The fold refers to the fold in a newspaper, distinguishing between what is seen straight away, and what requires the viewer to explore the page in order to see a given message.
In digital marketing, the fold has come to mean the end of the immediately viewable part of a web page.
'Above the fold' refers to an ad that is viewable without scrolling in the web browser. You don't have to scroll down (or sideways) to see it. 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 1024 by 768 pixels or at 1280 by 1024 or higher (or as low as 800 by 600!).