What is Google pagerank?

The Google pagerank link analysis algorithm that tells the status of a web page. In other words it tells us how much a web page is popular in the search engine giant google. The page rank is determined by an index number on a scale from 1 to 10. the more popular the web page will be, the higher it will get the pagerank index number and the webpage that is less popular will get a low page rank index number. The more the pagerank of a web page will be the more it will appear on the top of search results and vice-versa.

Google gives the index number to a web page depending upon the number of web pages linking to that web page i.e how much how much websites backlink that web page. The more the web page has backlinks the more the rank index it will get in the index scale. But not every backlink will increase your pagerank it depends on the page rank of the site that contains backlinks to the webpage. So, the webpage will have good page rank if it has backlinks on the sites that has a good pagerank.

An Explanation From Wikipedia :

Mathematical PageRanks for a simple network, expressed as percentages. (Google uses a logarithmic scale.) Page C has a higher PageRank than Page E, even though there are fewer links to C; the one link to C comes from an important page and hence is of high value. If web surfers who start on a random page have an 85% likelihood of choosing a random link from the page they are currently visiting, and a 15% likelihood of jumping to a page chosen at random from the entire web, they will reach Page E 8.1% of the time. (The 15% likelihood of jumping to an arbitrary page corresponds to a damping factor of 85%.) Without damping, all web surfers would eventually end up on Pages A, B, or C, and all other pages would have PageRank zero. In the presence of damping, Page A effectively links to all pages in the web, even though it has no outgoing links of its own.

Some more resources : http://en.wikipedia.org/wiki/PageRank