The Fold

The Fold is a term used to describe the portion of a page that is easily visible when users first open a page.  The area that encompasses “the fold”, is about 600px tall, and should contain the most important content on your site.

http://en.wikipedia.org/wiki/Above_the_fold
http://iampaddy.com/lifebelow600/

Google has recently chosen to state the importance of the fold by changing search rankings to value content above the fold more.  Google want users to easily locate the content that they were searching for when they follow search result links to a site.
http://googlewebmastercentral.blogspot.com/2012/01/page-layout-algorithm-improvement.html