A ‘Page Rank’ is a number Google gives to a web page that represents how important Google thinks the page is on the web. When one page links to another, Google considers it to be effectively casting a vote for the other page. The more ‘votes’ there are for a page across the whole web, the more important that page must be.
The importance of the page that is casting the vote determines how important the vote itself really is, meaning in Google calculations a page’s importance comes from the votes cast for it. These votes are then taken into account when the page is ranked.