Page rank is the position where your web/blog is placed on Search Engine Result Page (SERP). For example, type "Plasma Telly" on yahoo.com Search engine and you find
www.PlasmaTelly.com "ranked" in 1st position of yahoo.com SERP. But when typed the same "plasma telly" on Google's Search engine, you find the same site "ranked" in 6th or 8th position on Google's SERP.
When your site ranked in 1st position, that means your site can easily be found by those who searched for that particular keyword and that means traffic for your site and traffic means money. If your site is not well ranked, it means your site is not ranking to the keyword that makes up the domain and has no worth than the registration fee. Cheers.