This term is a carry-over from newspaper publishing days. In newspaper terms, “below the fold” means content was on the bottom half of the page (below the physical fold in the paper). In web design terms, “below the fold” refers to the content that is generally going to be below the point first viewable to the average website visitor in their browser (in other words, viewers would have to scroll down to see the content).
This is especially important to search engines like Google AND your user experience , for both, it’s important that the right content is ‘above the fold’, especially on mobile devices.