"Above the fold" is a newspaper analogy which describes the part of a web page which appears before you scroll down. There's plenty of research to show that a very large number of people arriving at a web page never see "below the fold". I've been conducting an experiment on our own website recently, replacing the whole site with a series of fairly unexcitingly-designed pages which have a single download on each page, a single "next step", and – crucially – nothing out of sight, "below the fold" for as many visitors as possible.
The results, in terms of pages-per-view for visitors, have been very impressive. A large number of people follow the narrative and view almost every page on the site. When there were loads of options on each page (many initially hidden, below the fold), it would appear that it didn't encourage people to click around the site. Instead, they left the site more quickly.
What constitutes "below the fold"? If you look at your traffic analytics, you can find the screen resolutions of your visitors. I tried this exercise recently on a consumer-oriented site I run, and found it had been viewed by 182 different screen resolutions!
Export all of this data into Excel, sort on screen height, and create a cumulative percentage column (Insider Programme members – ask me to do this for you if it's a chore!). Then find a nice cut-off point around the 90% mark, for example. For us it's clearly a screen height of 768-pixels.
Now, set your own PC's display to a resolution with this height, and take a look at your own website. Or if you use Firefox, get the Web Developer Toolbar, which resizes your browser window easily. The "fold" line for your cutoff percentage of visitors is no higher than the bottom of the screen. In our case this means that 93% of visitors can see at least this far down the page without scrolling. You really need to ensure that nothing falls below this which is critical to the "at a glance" message you wish to get over. If there's a download, a slogan or a testimonial which you'd like almost every visitor to see, it needs to be visible without scrolling at this resolution.