Pagerank, in plain vanilla terms, is the function of the incoming links to a site. That means that essentially, the higher the number of links the higher is your pagerank. Of course there are other factors like the pagerank of the page where your link is placed and the number of outgoing links on that page that affect the end value of your site’s pagerank.
However, the theory that higher page rank means higher rankings and better search engine placement is absolutely false. If that was the case, then Google, Yahoo, Microsoft, Apple and all other PR 10, 9 and 8 websites would be ranking for every keyword searched for.
Hence, it would be a folly to aim for just a Pagerank, without focusing primarily on optimizing the site for key terms that will result in quality traffic.