Below the fold is a term used to represent the area of a webpage that displays only after you scroll down.
The term "below the fold" was originally used in the newspaper industry to represent the area of the page that was below the fold in the newspaper. This would always be the bottom 50% of the page or in the case of the front page, it would be the part that you could only see once you had lifted the paper up and opened it.
There is no standard on the web for what officially constitutes "below the fold" since there are great differences in the size, shape and resolution of monitors. That being said, below the fold is usually defined as anywhere after the top 750 - 1000 pixels of a web page.