This displays a thumbnail of the entire topology that appears in the larger screen to the left, framed by the zoom level of the view. Click a location to move the larger view to center on it.
Use the slider at the bottom of this panel to change the magnification of your view. The icons to the right of the slider let you click them to fit visible icons vertically and both vertically and horizontally. You can also click and drag the cursor within this overview to change the magnification.