The term comes from the world of newspapers. The top half of the newpaper is above the fold and where the important bits go. The parts under the fold are less important because they aren't seen until people pick up the newspaper and turn it over.
In case you didn't guess, under the fold is a derogatory term that means that the information isn't seen until people actively take steps to see it.
In terms of a web page, under the fold means that which is not seen on the screen, commonly the parts of the website that are lower down and require the user to scroll in order to see them.