“Above the fold” refers to the section of a webpage immediately visible without scrolling. It’s a carryover from the days of newspapers, where the biggest story would go on the front page, above the fold in the paper.
Back when newspapers ruled the earth (yes, I’m unconsciously trying to make print media sound prehistoric), The Fold was a physical crease in the paper. Nowadays it’s a bit more complicated. You have different screen sizes and resolutions, toolbars that take-up room in the browser, and some people resize their browser window differently… There are a lot of factors.
http://yourdomain.com/#foldspy. An interactive overlay appears, allowing you too see what parts of your site are visible above the fold by different people. As you adjust the overlay, you’re shown a percentage based calculated by the viewport size data collected from all of the sites that use FoldSpy.