Microsoft Power Query provides a powerful data import and cleansing process experience While working with Power Query in Excel and Power BI M Query plays a vital role. The capability of Power Query is to play around raw data to combine, to filter, to append and many important task to shape up the data one wanted to have before loading into the data model. Power Query uses M Formula Language. It's a functional, case sensitive language.
But Easy! To understand few basics of Power Query and M Language watch this Video