'Zoom In' displays a smaller area of the image. Together with Zoom-out, the magnification of the image may be varied from 1% to a value that depends on the amount of system-memory available. You may also rotate the mouse-wheel to zoom-in.
The zoom value is displayed together with the Display Image Size.
(not shown on screenshots below)

Once you zoom sufficiently, so that you can only see part of the image, you can drag the image on the screen by keeping the mouse left button depressed.
Resampling ON/OFF allows you to choose between speed and image quality when zooming.