The pagerank google displays is mainly a fun measure, rather than a measure of much importance. But I still find it fun to look at the pagerank values - except when they go down for my sites :-( Now I also track the similar MozRank and can take some solace if the MozRank goes up :-)
Pagerank is a value given to the links coming into a web page on a logarithmic scale. So a PR of 2 is 10 times greater than PR 1 and 100 less than PR 4. MozRank is a similar measure, developed by a separate company that is updated much more frequently. See more details on this topic in my previous post: Google PageRank and MozRank of some of my pages (Oct 2011).
Google updates the visible PageRank occasionally (often about every 3 months). The real pagerank Google updates much more frequently (it is only the pagerank shared with the rest of us that is only updated occasionally.
Check the current pagerank on your sites using our related site: Multiple Site PageRank checker.
* internal pages
** new url, old url forwarded
- didn't exist yet
[blank] I don't know what the pagerank was, sometimes the site didn't exist yet.
Related: PageRank Updates for May 2012 - Web Page Authority - Google's Search Results - Should Factors Other Than User Value be Used