ATF represents search engine rank results that are above the fold; results that do not require the user to scroll in order to view them. If Above the Fold is an available option in the Rank Ranger report you're viewing then there is also an option to select a screen resolution from a list of the most common vertical dimensions.
This is an illustration of what users of Google Search might see above the fold depending upon their screen resolution.
In the above example, a person with a 1920 x 1200 screen resolution would see 2 advertisements, 1 featured snippet and organic positions 1 and 2, whereas a person viewing the results with a 1440 x 900 screen resolution would only see the 2 ads and featured snippet with no organic results above the fold.
Which ATF screen resolution setting to choose?
If you don't have a feel for the type of equipment that your potential audience might use, you could base your selection on what your current site visitors are using. Google Analytics offers a detailed report of screen resolution dimensions of visitors to your site, and you could start with that data to determine the setting(s) to choose in our reports. The Google Analytics report can be found in their Audience > Technology > Browser & OS section, you'll need to click the Screen Resolution option between the graph and table.
For additional information on this topic, Jesse Teske of Blast Analytics and Marketing authored a comprehensive article titled "Are Organic Search Results Still Above the Fold?"