Page Rank is link analysis algorithm, used by the search engines to determine the quality and quantity of the links present on the website. The site is given a rank between 1 to 10 with 1 being the lowest and 10 being the highest. Each website is re-evaluated 3-4 times a year.
The Page Rank of the site can be increased by building quality backlinks by directory submission, local listings, social bookmarking, blogging, etc.
The links present on the site – internal links, outgoing and incoming links should be relevant to the site and not coming from spammy and site having no connection with your site.