They're the google page rank (displayed in the google toolbar) and is a measure of a web page's popularity. Typically the home page will have the highest PR and the further into a site you go the lower it will get. Google updates PR infrequently and the formula (algorthm) is a well guarded secret. The best measure of popularity that you can manipulate is to get other sites to link to yours. How you get that to happen is up to you! Buying links is not recommended.