When I first started working in the website arena, Page Rank was one of the most important metrics to determine how important your website is.
Page Rank was originally developed by Google to evaluate how important a website is and it was the de facto definition for your domain’s authority. The scale went from 0-10 with 10 being regarded as the most influential.
Recently however there seem to be signals that page rank will no longer be updated, at least publically. The last one was in February of 2013 and it seems likely that Google will no longer be updating Page Rank at least for users to see going forward.
Whilst page rank itself has no real bearing on rankings, it is likely that higher page rank sites will perform better than lower ranked sites.
In the past to improve your page rank you need in theory to have links back to your site from other sites which are regarded as being an authority in the field.
As it appears that Page Rank is no longer updated another way of measuring your Domain Authority, which is perhaps more useful is Moz.com’s domain authority score. This goes from 0-100 and is updated much more regularly.
References: http://moz.com/learn/seo/page-authority
