Back in the day, if you wanted to show an image to a large audience, you would use an overhead projector. This would shine a light through a transparency, which is a clear sheet with a black drawing printed on it, and project the resulting shadows onto a large screen.
You are currently preparing a talk for the NSA, where you need to display some TOP SECRET information using an overhead projector. All of this fits on a single transparency, but you are worried it may fall into the wrong hands. For safety, you instead want to prepare several transparencies so that
when all of the transparencies are stacked, they display a legible message, but
if an adversary intercepted some, but not all, of the transparencies, they would learn nothing about this message.
How can you do this?
For our purposes, assume a transparency is an array of (quite small) pixels, each of which is black or clear. When several transparencies are stacked, the corresponding pixel on the big screen is black if at least one of the stacked pixels is black. Two black pixels in a stack looks just as dark as three, or any number.