There are a number of pre-built templates. Template previews will attempt to utilize your Site Setting configurations to create template previews that are accurate representations of how they will look in your site.
Page templates are designs for an entire page and can be utilized by creating a new page using "New..." in the toolbar:
Object templates are design for a particular object type, like modules or rows. They can utilized by clicking the + button and navigating to the "Add Template" tab: