From GeoGebra Manual
This tool is available only in the Spreadsheet View. There are two methods how to use it:
- Select a destination cell, then choose this tool and after that select a cell range. Sum of selected cells will occur in destination cell.
- Select a cell range containing more than one cell and then choose this tool. If the cell range has multiple rows, sum of each column will be inserted under this column. If it has only one row, sum of the row will be inserted to the right of the selection. If you hold Shift when clicking the icon of this tool, sum of each row in selected area is computed.