Some call it a 3D reference or a spearing formula. If your Excel worksheets are all perfectly lined up, you can use this formula to sum across worksheets in Excel.
But if your worksheets are not in the same sequence, then you can use TOCOL, SORT, UNIQUE, and SUMIFS to solve the problem.
This video answers these search terms:
how ot easy sum tabs in excel
how sum across multiple sheets in excel
how to calculate sum from other tabs in excel
how to do sum across two worksheets in excel
how to do sum of cells in multiple sheet in excel
how to get multi excel sheets to sum
how to get sum from multiple sheets in excel
how to sum across excel tabs
how to sum an entire workbook in excel
how to sum cells from different excel sheets
how to sum...