Slicer has two mouse modes: Transform (which allows interactive rotate, translate and zoom operations), and Place (which permits objects to be interactively placed in 3D or 2D).
The toolbar icons that switch between these mouse modes are shown from left to right above, respectively. Place Fiducial is the default place option as shown above; options to place both Ruler and Region of Interest Widgets are also available from the drop-down Place Mode menu.
Hint: Transform mode is the default interaction mode. By default, Place mode persists for one "place" operation after the Place Mode icon is selected, and then the mode switches back to Transform. Place mode can be made persistent (useful for creating multiple fiducial points, rulers, etc.) by checking the Persistent checkbox shown rightmost in the Mouse Mode Toolbar.