Flip on short edge comes into play when you’re printing a document in landscape mode and, again, want the document to read like a book, with page 2.
What does long edge mean in printing. 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. The edge refers to where the next page follows on from with your print remaining upright. It all depends on the grain of the paper.
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. Short edge means the pages are bound along the short edge of the page. I’m assuming you’re referring to printing presses, and not desktop printers.