Power Pivot is an add-in for Excel that originally became available with Excel 2010. It essentially gives you SQL Server Analysis Services running directly within your copy of Excel. What this allows you to do is create an in-memory data model using data imported from external sources as well as from worksheets within the Excel file itself.
Just like the data model we created in Chapter 3, Building Data Models, it is a collection of tables and relationships that can be expanded using DAX to create new columns and measures. However, unlike Power BI Desktop, you cannot use DAX to create new tables in Excel.