You'll see this term in a lot of interactive marketing books and papers. It just means something simple: the part of an email message that is visible without scrolling.

That's is.

Traditionaly it refered to a printing term for the top half of a newspaper above the fold. Content in this area is considered more valuable because the reader sees it first. 

Unlike a newspaper, email "fold locations" are not easy to determine because it depends on the users' preview pane, monitor resolution, or any toolbars in the e-mail client.

The opposite is "Below the fold", or the lowest part of the e-mail.