They're intended to be printed at high-resolution, then captured (photographed) at a distance where the fine lines at the center of the image greatly exceed the resolution capabilities of the capture device (usually a camera).

With a film camera, you could test the limits of the lens and film. With a digital camera, you test the limits of the lens and sensor.

The regularity of a pixel array in a camera sensor or display results in the Moire pattern you see in these test images.