miracle welcome and PR is page rank defined by Google. It is a numerical representation of the relative "ranking" of a page compared to all of the web pages in Googles index - not a specific keyword or anything
When you hear someone talking about a PR5 and wanting to trade - run the other way
People that use PR to base decisions on linking on dont have a very good grasp of Google and how it works - they are looking at the Google toolbar that can be downloaded at Google, and it has a little green bar that tells you the PR of any page.
Nowadays it really is pretty meaningless although PR is among a little over 100 things Google uses to rank a web page in the search results - hope that helps