The lower half of a newspaper when folded that receives less attention because its visibility is obscured, or in web design and advertising, that portion of a page that requires scrolling to become visible. In that case it is somewhat fluid because of varying screen dimensions that must be anticipated.
Generally content and advertising below the fold will receive less attention and lower click through rates that lower sales conversions.
Advertising rates therefore command lower rates if they are below the fold.
Contrast with Above the Fold