One of the key components of the Power BI service is the Data Management Gateway. The Data Management Gateway is a client agent that is installed within your on premise infrastructure to shuttle data from your on premise environment to Power BI running on Office 365.
Configuration of the refresh is done through the Power BI for Office Admin Center.
When the Data Management Gateway was first released, it was limited to fetching data from SQL Server and Oracle. With the latest version, all Power Query data sources which include a wide variety of data repositories including Hadoop, SAP Business Objects, Active Directory, Exchange, ODBC data sources, etc. are supported for refresh.