Duplex printing is printing on both sides of the paper. The printer must have an optional duplex unit installed in order to print on both sides of the paper. Check the documentation that came with the printer to determine if this option is supported.
If you have an optional duplex unit installed in your printer, you can specify duplex printing by indicating the binding edge you want to use from the Duplex options.
If you select Long-edge, the pages of the document turn like the pages of a magazine. If you select Short-edge, the pages turn like the pages of a legal pad