A proprietary Google technology that forms one part of how Google ranks sites in its search results.
Although PageRank is often perceived as completely determining Google’s ranking for a site, it is actually only one of many factors that the search engine takes into account. However, as it is the only ranking factor that Google publicly reports (although the accuracy and freshness of this report is often debated), for some people it remains an important metric in measuring the performance of an SEO campaign.
Notably, PageRank provides only a latent ranking, which does not necessarily correspond to the traffic levels that your site actually receives.
You can read lots more about PageRank here.