Above the Fold refers to any area on the web page that is prominently displayed and this generally is the upper half section that the visitors can read without touching the scroll bars of the browser.
It’s a common understanding that Google AdSense Ads perform best when placed above the fold as they get maximum attention and hence a higher CTR.
There’s however a different opinion now which says that Ads may get more clicks when placed next to the content or near the comments:
"A couple years ago there was this idea that putting the ad above the fold would get more attention. However visitors just completely overlooked the ads because they were coming to the site to read the content and didn’t give a doodle about what sat on the way top of the page. And really, when people were reading the content, the ads were too high to see and/or click on and website owners and their advertisers were not making any mula.
Today we put the ads next to the content or in between action points like commenting or sharing links. This is because the visitor subconsciously sees the ad and may be looking for something to do next. When next to an action point like a comment, the visitor is no longer reading the article and is much more inclined to click on an ad. " Link.