This option allows users to export PDFs in "Landscape mode".
When printing a document or creating a PDF, there are two common orientations: Portrait and Landscape. Portrait is very common, where the document is taller than it is wide. Landscape is when a document is created with the width being greater than its height.
This image may help: http://answers.umhb.edu/sites/answers.umhb.edu/files/images/portrait-landscape.gif