PR means Page Rank. This is how Google determines the importance of each page.
Every webpage that is indexed by Google is assigned a Page Rank from 0 to 10. The more important Google thinks the page is, the higher the page rank it gets. For example, Google.com has a PR of 10. Wikipedia has a PR of 8.
While no one outside of Google knows the exact algorithm used to assign a pagerank for each page, some people say that it depends on the number of backlinks you have and also where your links. For example, if you have your link on a page that has a higher pagerank, chances that it will increase your pagerank.
You can download the google toolbar for free and see the page rank for each page you visit.
Please note, that pagerank are updated every so often. So you are not likely to see a pagerank continuously changing.
Hope this helps.
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