The Preview option opens another tab in the browser to show what is viewed on the designer. If we use Firefox, by pressing the Control + Shift + M keys the browser will change resolution to 320x480 in order to emulate the appearance it would have on a Smart Phone.
The QR Code option opens the following screen with the QR code that may be scanned from a Smart Phone or Tablet, in order to open the URL of the site for mobiles in Preview mode and verify how it looks on the various devices.