You select multiple cells or rows the same way as in Excel: drag across the cells, hold Shift and click, or click a column header. To select individual rows that are not next to each other, hold Ctrl (Cmd on Mac) and click.
Ways to select
Drag: Click the first cell and drag to the last cell.
Shift-click: Click the first cell, hold Shift and click the last cell. Everything in between is selected.
The whole row: Click the row number in the leftmost column.
Ctrl-click: Hold Ctrl (Cmd on Mac) and click the row number of each row you want to include. If you click a selected row the same way, it is removed from the selection.
The whole column: Click the column header.
The whole table: Press Ctrl+A (Cmd+A on Mac).
The whole group: If the table is grouped, click the group row to select all the rows in the group.
Good to know
When you have selected multiple rows, the actions that apply to all the selected rows are shown at the top of the table. If you have selected cells, you see the sum, average and count at the bottom of the table.