Above the Fold
The origin of this phrase is in the newspaper world, where placing a story on the upper-half of the page was considered "above the fold" because of its location above where the newspaper was folded in half. This was considered prime placement because you could see it when newspapers are displayed for sale. "Above the fold" is now sometimes used to describe content on a web page that is visible without scrolling. This area may be attractive for both editorial and advertising content.
Readers are used to scrolling, however, so don't make a decision about placement assuming the most effective content is what readers see first. You should always test the placement of content and advertising on your website. Let the numbers speak for themselves.