![h-index](http://researchers.dev.lincoln.ac.uk/files/2011/03/h-index-300x274.jpg)
An example researcher would have an index of h if he has published h papers, each of which has been independently cited least h times. The bottom line here is that the h-index combines the number of publications and the number of citations per publication into a single metric.
The major caveat here is that it only works properly only for comparing academics in the same field since citation density differs widely among different fields.