The Microsoft Graph API is a service provided by Microsoft that allows developers to connect their applications to Microsoft cloud services such as (Sharepoint, OneDrive, Mail, Calendar, Active Directory etc).
https://developer.microsoft.com/en-us/graph
It took me a little while to understand and figure out how to integrate the Graph API into our own internal API services. To help you out, this article shows how I accomplished authentication and how i consumed some of the Graph API services within my own internal API services.