It's Google PageRank but it really doesn't mean anything. It used to be the main factor Google used to rank sites but it was so easy to manipulate they barely factor it in at all now. It basically reflects the raw amount of links flowing to any given page, but doesn't reflect any of the metrics that actually affect the quality of the links now.
Even though it hasn't been relevant in 5-7 years, people still use it as some indicator that a link may be good, but this has become less and less effective over time since people will just inflate their PR for the sake of either selling or trading links.
The best real measure you can use is how a site ranks for its target keywords. A site that ranks is obviously strong. Another option is using authority ratings from
http://www.majesticseo.com, which aim to be a single number like PageRank but reflects the metrics that actually matter.