DAX (Data Analysis Expressions) is a rich calculation language for use with Power BI.
While PBI allows data to be aggregated and filtered and pivoted with ease, DAX allows for more complex calculations like Year to Date, Ranking, % of Total, and much, much more.
As often mentioned, Power BI is a data model driven BI tool. The behavior of of reports is driven off a data model rather than by code. This gives us a lot of agility and a good
understanding of DAX will allow you to add analytics to your reports.
Dax Basics and Overview
Excellent free Dax class From 2 of the most knowledgeable people on MS BI.