In most advanced mobile devices, the orientation of a page changes based on how the device is held. When the user holds the phone vertically, the portrait view is displayed. When the user flips the device horizontally, the page readjusts itself to fit the landscape dimensions.
In Dreamweaver, the option to view a page in either Portrait or Landscape orientation is available in both Live View and Design View. Using these options you can test how your page adapts to these settings. You can then, if necessary, modify your CSS file such that the page is displayed as intended in both these orientations.