Above the Fold
Above the fold is what's visible without scrolling. It has to convince within a few seconds.
The term comes from print, where a newspaper's fold hid the bottom of the page. On the web, it refers to the screen area visible on arrival, before any scrolling.
This zone carries the main message: what the company does, for whom, and the call to action. A visitor who doesn't understand the offer at a glance often leaves without scrolling further.