Citation tracking looks at the number of times that a particular work, author or journal have been cited in the bibliographies of other works. This gives an indication of how they have been received by the academic community.
The branch of library science concerned with the application of mathematical and statistical analysis to bibliography; the statistical analysis of books, articles, or other publications. (Oxford English Dictionary Online).
Check this guide on bibliometrics.
Dr. J. E. Hirsch created the h-index and explained it in the paper "An index to quantify an individual’s scientific research output" in 2005.
If your h-index score is 24, that means you have at least 24 papers/articles that have been cited by at least 24 other papers/article.