DAX (Data Analysis Expressions) are expressions or formulas that are used in different BI and visualization tools for data analysis and calculations.
DAX is also known as function language, where the functions, operators, and constants are used to make one formula to get results. This helps the analysts to find new ways to calculate data values and bring more and better insights.
I recommend this Power BI Certification course by Intellipaat to learn Power BI.
Also, watch this video on DAX in Power BI: