A site-map is a simple text based page that displays the structure of your website, and consists of a hierarchical listing of links to all the pages that are available. Your visitors may visit this page if they are having problems finding pages on your site. While search engines will also visit this page - ensuring good crawl coverage of your pages - it's mainly aimed at human visitors.
On large old-school sites made of static webpages, these pages required manual updating, which goes part-way to explain why they didn't always accurately reflect the current's site's structure.
The site map is fully integrated into all of the modules, so links to private content available to registered users, only show on the site map when the member is logged in.
A site map's main benefit is to give users an overview of the site's areas in a single glance by dedicating an entire page to a visualisation of the information architecture.