Google of course reconfirms this on their Adsence Tool Site as you can read the text in the Picture to the left. Above The Fold is a Newspaper term where there is a physical fold in the newspaper itself where the stories and headlines attract the most attention, this is also true with websites.
There is one important rule to remember when placing AdSence on any given page
1. No more than 3 ads per page
No matter if it is a text Ad, Banner ad or Link Unit ad you must only use 3 per page. My suggestion is one in the header, one in the body and one in the footer, However I may use 2 in the body if it is a long article or page.
Check out The Above The Fold Adsence Tool and see if your main content can be seen above the fold as well as any ads you may have.