M Language (Power Query Formula Language) is a functional, case-sensitive mashup language used in Microsoft Power Query for data transformation and preparation across Power BI, Excel, and other Microsoft platforms. Built on a let-in expression structure, M enables powerful ETL operations including filtering, combining, and reshaping data from diverse sources. Understanding M's query folding capability—where transformations are pushed to the data source—is critical for performance, while its lazy evaluation and extensive function library (700+ functions) provide flexibility for complex data workflows without sacrificing efficiency.