Above the fold is a term used to represent the area of a webpage that displays on a standard sized monitor without having to scroll.
The term "above the fold" was originally used in the newspaper industry to represent the area of the page that was above the fold in the newspaper. This would always be the top 50% of the page or in the case of the front page, it would be the visible part that you could see while the paper was still on the newsstand.
There is no standard on the web for what officially constitutes above the fold since there are great differences in the size, shape and resolution of monitors. That being said, above the fold is usually defined as anywhere within the top 750 - 1000 pixels of a web page.