Question : What is meant by 'PR' of a website ?

Answer : Page Rank of a website is a numeric value that tells as to how important a web page is on the internet. Google's view point is when one page of a website links to another page, it is effectively casting a vote for the page to which it links. The more the votes or links that are casted for a page, the more important the page becomes in the eyes of search engines. Also, bear in mind that the weightage is given depending upon the importance of the page that is casting the vote. Google calculates a page's importance from the votes that are casted for it. While calculating Page Rank it is taken into account as to how important each vote is in context to its own value/weightage..
PageRank is Google's way of calculating a webpage's importance. It is one of the major factor which determines a websites ranking in the search results, though it isn't the only factor that Google uses to rank pages, but it is MOST important one.

'PR' is nothing but short form of PageRank.