PageRank is an algorithm used by Google for link analysis. It was invented by Larry Page and Sergey Brin; the owners of Google. It was developed by Google to rank pages on their merit and not on meta tags as people had started misusing meta tags to improve their rankings.
It is an important off-page optimization factor. It decides how easily and quickly your users can find your webpages on the world wide web. If your PageRank is good the user will find you easily as the search engine will keep your site higher on the search engine listings.
Some important link building strategies to improve your Google PageRank: