Have you ever heard the phrase 'above the fold'? Have you ever used it without knowing what it means or find others don't know what it means?
For website design, "above the fold" is the idea that the most important or actionable information for that webpage should be visible without people having to scroll down. This may include phone numbers, headlines and other forms of call to action.
The problem for website design is that screen sizes vary drastically so there is no set definition for the number of pixels that define the "fold". Then to further complicate designs, responsive websites adjust their layout based on the size of the browser window/device, so the fold is not a static feature of the page.
Where did it originate?
Above the fold originates from the newspaper industry where positioning a story or graphic “above the fold” on the paper could increase readership. Since newspapers are displayed and sold flat - if the headline or photo was compelling it could increase sales. And of course, readers are more likely to read and be interested in things placed on the top half of the front page.
Is it still relevant?
Really, the 'fold' does not matter anymore. It is in an entirely different place on a desktop, phone and tablet, on an iPhone vs. an Android phone, on one tablet brand vs. another, and even on a phone or tablet in landscape mode verses portrait mode.
Nevertheless, the concept of "content that is visible on initial page load" is still meaningful. It is just that now you need to consider the "fold" separately for different kinds of devices.