In online advertising, "above the fold" refers to the content that is visible on a page without the user needing to scroll down. It's derived from the newspaper industry, where "above the fold" referred to the top half of the front page, visible when the newspaper is folded.
You can learn more about viewability best practices, which explains page length and ad placement for Above-the-fold (ATF) and Below-the-fold (BTF) placements.