PR is a measure of how important a page is (according to Google). For the purpose of this article, I will only discuss PR in the SEO context what it means and how to use it.
First things first: PR is not the cause or the result of high rankings.
PR depends mainly on 2 things: Incoming links and traffic. Some SEOs make the mistake of assuming high rankings come as a result of high PR, when in fact high PR is one natural outcome of high rankings.
Better SEO = higher rankings = more traffic = Higher PR
However, the concept behind Googles PR is crucial when working with External Links (i.e.: incoming) and Internal Links (i.e.: the websites structure). Every link from A to B is considered as a vote of confidence/credibility from A to B. We can use PR as a physical measurement of how this credibility travels from one URL to the next.
The role of SEO in this case is
Off-site: to maximise incoming PR by increasing the number of incoming links and the quality of incoming links.
On-site: to harness incoming PR so that none of it is wasted (on broken links) and that most of it is channelled to the most important pages of the website (i.e.:...