There will be a list of all of the pages on your site. There will be one section for each category of pages. To change the order in which pages are listed, simply select pages by clicking on them and then click on the up and down arrows.
 
This is useful if you want to use automatic navigation and you want to have control over the order in the links to other pages appear your the site.
 
Once you have finished, click on the "Store Changes" button.