In terms of SEO and SEM,
Page Rank refers to where a website or web page ranks within search engines. For example, if a website deals with cars, when a person performs the search for “cars” in a search engine, their ranking indicates where in the search results of their web page they appear in the list (for example, within the top 5 results, on the first page, page 300 and so on). The rank of the Web page in a search engine is also commonly referred to as a position.
Page Rank is an algorithm used by Google Search to rank websites in its search results. Page Rank was named after Larry Page, one of the founders of Google.
Usefulness of Page Rank in SEO
Currently, while it gives some guidance on the popularity of a page, it is a metric that has become obsolete and that Google has stopped updating and displaying.