When the Zoom tool is active, you can use it to select an area of the image you would like to focus on by clicking on the image and dragging to define the area of interest.
Once zoomed in, you can pan around the frame by holding down the shift key, clicking on the movie, and dragging the mouse.
In this mode, the cursor will change from a magnifying glass to a hand.
You can also find the Zoom tool from the ‘View’ menu or using the keyboard shortcut.