Above the fold: Any area of a web page that can be viewed without visitors having to scroll. Ad space in this area tends to have a higher value than that located elsewhere on the page.
The area of a web page that can be seen without any scrolling or clicking is the area that will receive the most attention. From a strategy standpoint, this is the most important part of the screen. All the most important, engaging, informative, etc… stuff should be located on this part of the page.
Most visitors to a site are unlikely to spend a lot of time on a page if they don’t find something that grabs their attention right away. They shouldn’t have to hunt for useful information.
Simplicity and quickness are key here.