The extra page feature of the software allows you to insert a new "flat page" for your sites use that does not need to be created outside the software.
Highlights of the Extra page:
There is a support wiki article already that describes the default extra pages that we include within the default design here if you wanted to re-purpose some of those pages. But this article will cover the basics of creating your own extra page from the extra pages that currently do not have a use by default.
There are 4 basic steps to creating an extra page covered in this section of the support wiki:
1) Create the extra page template
There's already an article on rewriting extra page urls to be more SEO friendly here in the support wiki.