The h-index is a factor determining both the quantity and quality of a scientist’s research output.
The h- index is based on a list of publications ranked in descending order by the number of actions these publications received.
The value of (h) is equal to the number of papers (N) in the list that has N or more citations.
For example,
An author with 10 papers, where 5(N) papers were cited 5(N) times each one the h-index is 5.
If 8(N) papers were cited 8(N) times each one then the h-index is 8.
Hope, This will help you to find how the Journals and abstract indexing sites calculate Impact Factor(JIF) for a publication and H-Index for an author.