PageRank is a metric, developed by Google, to score a specific page. In the past PageRank played an important role within the ranking algorithm of Google. PageRank is scored using a 0 to 10 logarithmic scale.
Today the role of PageRank is far less significant, especially since Google PageRank updates are pretty scarce. A HIGH PageRank is still an important indicator to measure the strength of a page.
Update Oct 7, 2014:
John Mueller, Senior Webmaster Trends Analyst at Google, announced in a Google+ hangout video Google won’t be updating Google Toolbar pagerank in the future. Checkout the video for further information.