Pagerank is the proprietary method by which Google determines the relevance of your site to search results. The higher the Pagerank, the more often your site will appear for relevant searches.
I noticed today that the Pagerank of letterneversent.com is 4/10, while the PR of www.letterneversent.com is 5/10. That’s weird, but I guess Google does that because you can’t just assume that the “site.com” is the same as “www.site.com”. Occasionally, this is not the case.