Hello,
It’s a system Google developed. Exchanging links with sites that have a higher page rank than yours will improve your page rank.
PageRank™ – A system for ranking web pages developed by the founders of Google. In essence, Google interprets a link from page A to page B as a vote, by page A, for page B. It also considers the page that casts the vote. Votes cast by pages that are themselves “important” weigh more heavily and help to make other pages “important.” PageRank is scaled from 0 to 10 with 10 being the highest and 0 the lowest (no PageRank). PageRank is measured by downloading and installing the Google Toolbar. When you visit a web site a bar graph on the toolbar will indicate the PageRank value.
Yahoo has a PageRank of 9.
Cheers