When Duplex printing, you can choose which way you would like your print to appear on each side of the paper using the Short Edge and Long Edge features. Long Edge means pages are bound along the long edge of the page (left edge for portrait, top edge for landscape). Short Edge means the pages are bound along the short edge of the page.
The edge refers to where the next page follows on from with your print remaining upright. For example, long edge will mean that you turn from one page to the next along the long edge of the paper, much like an A4 magazine or standard brochure.
Short edge would mean that you turn the page from the short edge of the paper, much like that of a notepad.