The most valuable real estate on a website is the top part of the home page - what can be seen on someone's screen without scrolling down.
Just like the top half of the front page of a newspaper, it is the first thing someone sees.
And if it doesn't give the right impression, it may be the last thing someone sees on your website.
By analogy with newspapers, the term "above the fold" has come to mean that top part of your home page.
The first question of course is how much of the home page IS above the fold? As covered in my last couple of posts, that depends on screen size and resolution. And also, how much of their screen is taken up by tool bars. So there is going to be a lot of variation. With most screens it is going to be at least 600 pixels.
It is critical to view this part of your website as though you were a visitor, one who could be a serious prospect, and see if it does give the right impression.
Many websites waste an enormous percentage of their potential by doing this poorly.