Portrait mode is the orientation of the page that prints the image vertically across the page instead of horizontally. By default, all programs have the page orientation and printer pages setup in portrait mode. The image is an example of the printer paper orientation set up through the printer options in Microsoft Windows. As you can see, the top example is a paper orientation selected as portrait and the bottom is selected as Landscape.
When in portrait mode, the page is taller than it is wide. When in landscape mode, the paper is wider than it is tall.