The sheet tab of the current sheet is always visible in white in front of the other sheet tabs. The other sheet tabs are gray when they are not selected. By clicking other sheet tabs while pressing CommandCtrl you can select multiple sheets.
You can use Shift+Ctrl+Page Up or Page Down to select multiple sheets using the keyboard.
To undo the selection of a sheet, click its sheet tab again while pressing the CommandCtrl key. The sheet that is currently visible cannot be removed from the selection.
You can refer to a range of sheets in a formula by specifying the first and last sheet of the range, for example, =SUM(Sheet1.A1:Sheet3.A1) sums up all A1 cells on Sheet1 through Sheet3.