Google PageRank is a mystical creature that is a major attribute of how successful your site is found by Google searching.  It's basically Google's rating on the importance of your site.  Largely this is done by "qualified" inbound links to your page.  The more "qualified" links, the better.

Ian Rogers has some good information on explaining what it is and how it works.  It also provides some good references on the original research by the Google guys. 

If you are a webmaster, this topic is well worth your time to master.

Read: Google Pagerank Explained