There are many ways to measure the "output" of a researcher. It is likely there are many ways to measure it because none of them is right for all the cases. While many consider citations of a paper as some type of endorsement of other person's work, there many ways to turn that into numbers.
And we want numbers so we can compare performance easily. I was asked the other day to provide my H-index. Problem was I have never heard about it before (only my fault). Just a wikipedia query later it turned out the idea was quite simple. H-index was proposed by Jorge E. Hirsch from UCSD and it is sometimes called Hirsch index or Hirsch number too.
This is how it is obtained: Take your publications list and sort it by the number of citations. Now start counting papers from the top (most cited paper) and continue down till the number of citations is lower than the count of papers. The current count (or one less) is your H-index.
So an H-index of, let's say, ten means that person has published at least ten papers with ten citations (or more) each.
The main problem with this index is that citations are not always counted the same. For example, Google Scholar use to report more citations than Scopus or Web of Science. Number of authors is ignored by H-index, and it somehow limits the maximum H-index to the number of your current publications, no matter how groundbreaking they are.