What is PageRank?
PageRank is the term Google uses to determine the importance of a site. Other search engines might call it something different, but it all boils down to how much trust they have in your content and whether they believe a page should be included in the index. PageRank is a number derived from a mathematical algorithm.
PageRank applies to pages. A site's authority can be determined by adding up all of the pagerank that it's individual pages have. If a site has a page that isn't in the index, that page has no pagerank.
Not every page on your site will have the same amount of pagerank. The more incoming links and pagerank you have flowing in, the higher level of importance will be placed on that specific page.
Outgoing links matter too. The more outgoing links you have on a page, the more pagerank will flow off that page to the ones it is linking to.
This means that getting links from pages that do not have a lot of links is worth more than a page that is stuff with them. If two pages have the same amount of pagerank, one with 10 links and another with 100, the page with 10 links is roughly 10 times as valuable as the other. You get 10% of the pagerank passed compared to 1%.
You can modify your site to manipulate pagerank. You want to do this to get more pages indexed and to get a higher number for your most important landing pages.