From a design standpoint, it means that your site’s visitors will see the same navigational element on every single page.   This is most often done via a top or side nav.  For example, this website uses a right-side common navigational element  that is the exact same thing no matter what page you’re on.   There is also a less prominent TOP navigational element that exists for the homepage and About pages.

From a programming standpoint, it means that if possible, the same file should be used on every page for navigation.  Sometimes called “Include” pages, the navigation is placed in a separate file from the rest of the site.  That way, if you have to update a navigational link you only have to do it one place instead of on every single page.

From a load-time standpoint, this helps because the navigation typically only loads once and that bit of code doesn’t have to load on every click.

Here are some varied resources on the subject of site navigation:

http://articles.techrepublic.com.com/5100-10878_11-1058652.html

http://www.poynterextra.org/eyetrack2004/navigation.htm

http://searchenginesmarketer.com/2008/11/14/website-navigation-left-vs-top/