Newspapers used to be bought from piles at news stands.
Because the broadsheet format was so large, they were folded in half with their main banner and headline facing the top of the pile for easy viewing and a quick purchase by the consumer presumably in a rush to catch the peak hour train to get to work.
Here lies the genesis of the term ‘Above the Fold’.
The website equivalent is to put your main content and Call to Action ‘above the fold’ – somewhere onscreen that a consumer can see it without the need to scroll or ‘turn the page’.
ps. Bonus points if your Call to Action passes the ‘Squint Test’. That is, if I were to squint my eyes and look at your page, can I immediately tell what you want me to do?