DAX (Data Analysis Expressions) is a formula language used in Power BI development to create calculated columns, measures, and custom aggregations. It allows developers to implement business logic and calculations that go beyond basic data visualization. DAX helps in creating dynamic and complex calculations that drive key insights, such as year-over-year growth, ranking, or trend analysis. Mastery of DAX is essential for Power BI developers to optimize report performance and deliver highly customized, interactive reports tailored to a business’s specific needs.