In security printing, void pantograph refers to a method of making copy-evident and tamper-resistant patterns in the background of a document.
Typically they spell out "void", "copy", "invalid" or some other indicator message.
[1] Void pantographs work by exploiting the limitations and features of copying equipment.
A scanner or photocopier will act as a low-pass filter on the original image, blurring edges slightly.
This means that if a grey region consists of a grid of very small dark dots the filtering will produce a lighter grey, while a region of larger dots will be affected differently ("big-dot-little-dot").