Google ranks pages by assigning a numeric value (1-10) known as "PageRank" or "PR". The PR represents how important Google feels your web page is to the world. So the higher the PR value the more your site shows up in Google's searches.
Google calculates this number by counting the pages that link to your page. They also take into account the quality of the page linking to you. So the higher the quality of the pages linking to you the higher your PR.
You should track your page's ranking. There are several good Google PageRank Checkers you can add to your pages. I don't see why users need to see the PR, so I recommend only displaying them for site editors and/or editors. I prefer using Google Toolbar with the Page Rank turned on. This allows me to see not only my page ranks, but the rank of the sites I surf.
You can get detailed explanation of how Google Page Rank is calculated on Wikipedia.