Page Templates are files that you can create and apply to any or all of the Pages on your Site to make it easy to consistently apply content that is the same across your Pages.
For more information see: Site Manager - Page Templates.
How can I apply a Page Template to my Page?
When viewing Page Details, scroll down until you see the "Page Template" field.
This is a drop-down menu from which you can choose an existing template. Pick one and click save to ensure it is applied.
If no templates are showing in the drop down, you can find out how to create a template here.