An h-index measures the broad impact of an individual’s work and is a method to compare authors within a discipline, especially in the sciences.

Dr. J. E. Hirsch created the h-index in the paper;

J.E. Hirsch, An index to quantify an individual's scientific research output, Proc. Natl. Acad. Sci. U.S.A.
102 (46) 16569-16572, https://doi.org/10.1073/pnas.0507655102 (2005).

He asked and answered "How does one quantify the cumulative impact and relevance of an individual’s scientific research output?"

A scientist has index h if h of their Np papers have at least h citations each and the other (Np  h) papers have h citations each. 

If your h-index score is 57, that means you have at least 57 papers/articles that have been cited by at least 57 other papers/articles.

Your h-index can vary depending on the following points;

• The h-index value is based on a list of publications ranked in descending order by the Times Cited count.
• An index of h means that there are h papers that have each been cited at least h times.
• The h-index is based on the depth of years of the database you are using and your selected timespan.
• Source items that are not part of your subscription will not be factored into the calculation.