Google Scholar Metrics is designed to help authors "gauge the visibility and influence of recent articles in scholarly publications." Towards that goal, Google Scholar Metrics assigns publications a variety of rankings based upon Google's h-index.
H-index is based on h-core, which "is a set of top cited h articles from" a given publication. "The h-median is a measure of the distribution of citations to the h-core articles." Search results display h-index and h-median scores for publications based on the last 5 complete calendar years.
For more information on the methodology and data sources behind the rankings, see Google Scholar Metrics Questions? page.