The idea is to make sure our web page size fits most folks screens.
There are 2 dimensions to take note. NORTH - SOUTH and EAST - WEST
EAST - WEST
This is width of the page and is marked by the Vertical line on the right. Back in the day (before flat panels got cheap), monitor sizes varied greatly, you needed to make the site far narrower back then. These days, the East-West size is not a real design limitation. If you have a center justifyed site, slowly shrink the size of the browser until it contains the nothing but the page itself (not the background).
NORTH - SOUTH
This is the money maker (or money killer!) This is the horizontal line on the overlay. Any Content on your page that is below this line is not seen by the majority of shoppers and needs to be scrolled down to see it. This line has a knick name from the newspaper biz, it's called "the fold".
The reality is that most folks dont scroll down the page unless they land on a page that really hits them... then, they scroll lower. Additionally, repeat visitors will scroll down more often that 1st time visitors.
So, what's below your fold?
Are your lead generating tools below the horizontal line? Are your pics below the fold? Ohoh. not good. Yes, your right, not everything can be :above the fold". Does your page invite the viewer want to scroll down and discover more content?
UsedCarKing.com is far from perfect, but it was built to fit as much of the good stuff "above the fold" as possible, and leave the "below the fold" content for folks who want to be on the page a while.