Selecting objects
From GeoGebra Manual
To select an object means to click on it with the mouse after selecting the Move Tool.
If you want to select several objects at the same time, you could draw a selection rectangle: Select the Move Tool and click on the position of the first corner of your desired selection rectangle. Hold the left mouse key pressed down and move the pointer to the position of the diagonally opposite corner of your desired selection rectangle. After releasing the mouse button, all objects within the selection rectangle are selected.
Note: By holding the Ctrl-key (Mac OS: Cmd-key) while clicking on different objects, you can select several objects at the same time.