Why this matters
DAX (Data Analysis Expressions) is Power BI's formula language, and Power BI adverts and the Data Analyst certification expect you to know the basics. You do not need advanced DAX for a first job. You need a few clear measures, named well, that give the right answer whatever the user clicks.
What a measure is
A measure is a formula that calculates a single answer, such as total sales, based on whatever is currently filtered. Put Total Sales on a card and it shows the grand total. Put it in a chart by branch and it shows each branch's total. Click "Wa" in a slicer and it shows Wa's total. The same formula gives the right answer in every place, because it is calculated at the moment it is shown, using the current filters. Those filters are called the filter context.
Writing your first measures
Select your Sales table, choose New measure on the Home tab, and type in the formula bar:
Total Sales = SUM(Sales[Amount])The name comes first, then =, then the formula. Sales[Amount] means the Amount column in the Sales table. A few more: