Kentico CMS comes with automatic support of the Google SiteMap Protocol. This is a protocol designed to help search engines with the indexing of your site and can improve your site's position in search engine results. Simply put, a Sitemap is an XML file where URLs of your site are stored. Detailed information about the SiteMap Protocol can be found at http://en.wikipedia.org/wiki/Site_map.
Kentico CMS creates your site's sitemap automatically. Physically, it is located at <domain>/CMSPages/googlesitemap.aspx. It can also be accessed through the URL specified in Site Manager -> Settings -> URLs and SEO -> Google sitemap URL. This value is set to googlesitemap.xml by default, which means that the site's sitemap can also be accessed through <domain>/googlesitemap.xml. This is the URL that you enter into google when registering a new sitemap.
Please note that if you want the sitemap to be accessible via a URL with the .xml extension, you need to configure your IIS to handle the 404 error through Kentico's handler404, as described in the Installation and deployment -> Additional configuration tasks -> Custom URL extensions and extensionless URLs chapter. In this case, it is also necessary not to set the .xml extension to be processed by .NET.