A placement is a property that groups together Ad Unit in order to facilitate targeting
It's used to group pages that share similar content.
When naming a placement, choose to either:
Above the fold means that the location will be seen in the browser after the page load. The user does not need to scroll down to see the ad.
It's a reference to the fold of newspaper where what you see is the browser. Example:
Advertisers find ad placements in several ways, including:
If you target the same ads to serve on many different ad units, instead of adding all the different ad units to your targeting criteria, you can group those ad units into one placement and then just target that placement.
See Ad - Targeting