It is a window with an image of the solved puzzle. You can enlarge the image view or you can cut out only a certain part of it and enlarge this part. You can gain a more detailed view of the image then.
Enlarging
Drag the edge of the window. To specify the extent
of enlarging more precisely press the right mouse button on the
window.
Cutting out
Press the left mouse button and by moving the
mouse cut out the part of the picture you need. A new window
where you can see only the cut part will be created.
You can repeat enlarging and cutting out and create more windows.
Hot key: V (displays or hides all windows with an image view)