Before websites were around “above the fold” was a newspaper term. It meant the part of the newspaper that you could see when it was folded in half. So above the fold used to mean what was on the front page of the newspaper above the fold line.
For websites what this term means is the portion of your website that’s visible without scrolling down. This area of the website is important because people move VERY fast on the Internet. If you don’t snag people’s attention quickly they move on to the next website. So if you’ve got something important make sure it has maximum visibility by getting it above the fold!