PageRank (PR) is a measure of your sites importance. Although not entirely accurate at times it can give you a general idea about the health of your site. Page Rank is dependent on how many incoming links point to a given site or even page within a site.
A Page Rank meter is available within the Google Toolbar so you can dynamically see you and your competitors PR easily. The Toolbar is available for download at http://toolbar.google.com/.
Usually Page Rank is highest among the index page and then from there is diluted into the interior pages that link from the front page. Many webmasters are careful as to how many links leak out Page Rank from important and high ranking pages.
Having a high Page Rank (4 and above) does mean that Google will crawl it more often that ones with lower PR. This may be important for you if your site contains lots of fresh content such as a news or info based site.
The idea is to structure your site in a manner that parent pages (the ones with the highest Page Rank) feed the children pages below it. Although Page Rank is a important factor on how Google ranks your site, its only 1 of many factors within the Google algorithm.
Additionally sites that have high Page Rank and point to your site can boost your page rank as well. In theory, if you had only high PR sites reciprocating a link to you then eventually your PR will reflect your link partners PR.