As you begin to learn Power BI, you will inevitably encounter the term “DAX.” It stands for “Data Analysis Expressions,” and is a collection of constants, functions, and operators developed by Microsoft and used in select BI and data visualization tools. DAX is also called a function language because the full code resides inside the functions.
DAX is used to manipulate data and helps you define the custom calculations used in calculated fields and calculated columns, working with relational data, and performing dynamic aggregation. Put differently, DAX enables users to create new information from data that already exists in your model.
DAX uses some of the same functions as Excel, so if you are familiar with the latter, you may find DAX easier to use. It offers a higher degree of flexibility, letting users create formulas that are applied on a row-by-row basis.
Note that while DAX offers greater strength and flexibility to your data models, it runs the risk of making them overly complicated. Also, DAX comes with a steep learning curve, so be sure you want to work with it before committing.