In a web design context, your above the fold content is what your reader sees without having to scroll down. How much is above the fold depends on the reader - people have different sized screens and different resolutions. I see a lot less on my little EeePC screen than a gamer with a high definition 1920x1080 monitor. But in general, a header and some blog content are going to be 'above the fold'.
The term 'above the fold' actually has its origins in newspaper publishing - important stories and photos were placed above the fold - on the top half of the folded newspaper page. When you're first handed a newspaper, you look at the top half first. It was an easy transition from newspapers to websites, since the top area of a webpage is the first thing the reader sees.
~Joy
