The Source Image Viewer is used to select the locations of reference points within the source image to be georeferenced. The Cursor bar at the top of the viewer displays the current location of the cursor relative to the source image, in pixels.
You can access an additional Overview Map sub-window by selecting Window>Source Data Viewer Windows. The Overview Map sub-window displays a smaller view of the current source image as well as an outlined area that indicates the current extents of the main view window.
You will recognize most of the common control buttons from typical Windows GIS data viewing programs and can be reminded of their function by mousing-over any of the buttons for a pop-up description. There is one georeference-specific button on the viewer, the Make a New Point button. With this button selected, clicking anywhere on the source image automatically sets the Reference Point List control’s Source (Across) X and (Down) Y values to the clicked image pixel coordinates.
You can control various Source Image Viewer preferences via the Georeferencing Preferences.