DAX stands for Data Analytical Expressions. This is a language in itself that can be leveraged to do all analytical calculations, be it regular aggregations like SUM, AVG etc. or period calculations like YTD statistics or rolling numbers like six months averages etc or many others.
DAX is supported in all Microsoft technologies running over xVelocity engine like PowerBI, Power Pivot and SQL Server Analysis Services Tabular Model. This is one of those components that makes PowerBI an analysis tool rather than just a visualization tool.