Pagerank is an older formula Google used to judge the value of a page. The value of each page was determined by the quantity and quality of the pages linking to and from it. For example, if you have a page on sockets, and many people link to your page because it answers all their socket-related needs, that page will be judged as having high value or high PageRank. But this is a two-way street. Your outward links also contribute to your Pagerank.