If a display ad appears “above the fold,” this means that the ad:
- can be viewed in the upper portion of the page without scrolling
- will appear at the top of each page of the website
- takes up more than 20% of the webpage
- will appear anywhere on the front page of the website
Explanation: “Above the fold”, or the top fold of the page, refer to the portion of the page a user can see without scrolling down.
Read more here: https://support.google.com/adsense/answer/132618