What feature on a printer must be enabled so that a printer can print on both sides of the paper?

First, for a printer to print on both sides of the paper without you having to physically flip it over after it's done printing on one side, it has to support Duplex printing. This can often be found out just by looking on the printer,

Example: HP LaserJet M3035dn <-- where the d signifies duplex and the n is network.

If there isn't anything like that on the printer, just check the manual :D

Second, the printer driver on your computer has to have this feature enabled.

If it is, when you're in a document and are about to print, a window pops up with the printer and some more into. Besides the printer name, in most cases, there is a button that says Settings, most of the time. Click that, and choose duplex printing.

On many old style printers, it can say: Manual duplex, you have to turn the page. (Or something along those lines)

For a more in depth walkthrough, read the manual that came with the printer :)