In Kig, objects can be hidden. This is done by selecting the objects, mouse button clicking one of them, and selecting in the context menu that appears.
To unhide the objects, use the → . This will unhide all currently hidden objects.
Night Vision is a particular way of working with hidden objects. When you have to move or change something in one or more object but without unhiding all the hidden objects you have, then the night vision mode will be of benefit to you.
Basically, it allows you to see the hidden objects as if they were visible, so that you can manipulate them as you would normally. In Night Vision mode, the hidden objects will be visible with a grey colour.
To toggle the night vision mode, use → .