So far Power BI has summed columns for you. Those are implicit measures. An explicit measure is one you write in DAX, name, and reuse anywhere:
Total Sales = SUM(Sales[Sales Amount])
Select New measure on the Home ribbon and the formula bar opens with Measure = . Replace Measure with your name, write the expression after the equals sign, and press Enter or the check mark. As you type, a suggestion list offers functions, tables and columns; press Tab to accept one. The new measure appears in the Data pane with a calculator icon.
Measures can build on other measures: Profit = [Total Sales] - [Total Cost]. Write columns as Table[Column] and measures as [Measure]. Explicit measures keep the logic in one place, and the DAX in the next lessons needs them.