Landscape mode is a horizontal orientation of a document or image. Landscape mode is commonly used to print charts, or other images and text that may not fit properly if the page is oriented in portrait mode (vertically).
Pictured is an example of printer options where the user can choose how the printing should be oriented. The top selection is portrait (vertical) orientation, which is taller than it is wide. The bottom selection is landscape (horizontal) orientation, which is wider than it is tall.
Most software offers a "page setup" option in the file menu, document settings, or preferences, where orientation can be chosen.