The short answer is "Not much of anything". The little green bar in a Google Toolbar (Google Page Rank) used to be a real measure of how well a website would rank for a search term, well that went away some time ago. Today the Google Page Rank is almost worthless in terms of anything relating to how a web page or website does in a search. The page rank was for some time related to the number of back links from other web sites even though it was hard to define how many backlinks Google was counting.
There are much better tools to define how well a website is optimized for a given keyword phrase. Hint Google: seomoz