PR stands for Page Rank. The higher your PR, the more important it is in the eyes of Google. Go here if you wanna read a wall of text about it.
Google PageRank of page. It can from 0 to 10. Higher pagerank gives small part of your rankings, so link with higher pagerank is more valuable than low pagerank. You can read more at http://en.wikipedia.org/wiki/PageRank P.S. I don't believe much in PageRank, as there are much more things to put energy to increase your results in search engines
A lot of webmasters consider Page Rank to be very important. The higher your site's Page Rank the more you can sell ad space, links or the site for. It's like the Neilsen ratings of the internet.
Google Toolbar An example of the PageRank indicator as found on the Google toolbar.The Google Toolbar's PageRank feature displays a visited page's PageRank as a whole number between 0 and 10. The most popular websites have a PageRank of 10. The least have a PageRank of 0. Google has not disclosed the precise method for determining a Toolbar PageRank value. Google representative Matt Cutts has publicly indicated that the Toolbar PageRank values are republished about once every three months, indicating that the Toolbar PageRank values are historical rather than real-time values.[6] Taken from Wikipedia. I guess it's like respect, but it's more of the importance apparently.