Understanding required

Im getting good at these help me threads, yet slowly but surely im building up my understanding of the subject ( I hope). Anyway one thing which I still need help with is PR (page rank). Now I have found a tool here:

http://rankwhere.com/google-page-ran...net%2Fforum%2F

Which gives my site a Page Rank of 4. What does this mean? The word to me means the placing you will recieve on search enginees such as google but you will notice on google http://www.mirzone.net/forum/ is not even listed! Where as a site which is number one in the rankings is number one on most of the key words I want. Therefore could someone provide me a simple understanding, how is PR calculated and what is the scale between 4-5 for example meaning that instead of being in rank 1 I am not even listed on google for my main url (I have 132,000 supplemental results)

PageRank is essentially a measure of inbound linking to your pages. It is just one factor in Google's determination of the SERPs.

The PR you see in the Google toolbar is now always up-to-date, as this information changes behind the scenes and is only export to the toolbar after a period of a few months (exact schedule not published).