I thought this was just the distance you were from the first result on Google for a given set of search times. It’s not.
This is a number between 1-10 ( believed to be a log scale ) that scores your site based on incoming and outgoing links to relevant sites. The better ranked a place is that links to you or you to it, the higher score you receive.
Page Rank Explained gives a detailed analysis of how page rank is believed to work. Google for obvious reasons keeps this information private.
You can find the rank of your pages by either installing the Google toolbar on your browser and letting the mouse hover over it while you are on your page. Or you can go to Google PageRank Checker and type in your url.
It has also been rumored that the length of time your site has been in place and the future length of time for which you have registered your domain matter as well.