Photographs that are to be printed in books, magazines, newspapers are converted to a "half-tone". The most common way this was done:

A finished photograph on paper was supplied to the printer. This photograph was copied using a giant camera, common to all shops that prepared photographs for publication.

The camera contained a screen, not unlike window screen. These hovered just over the film in the camera. A copy was made on photographic film. The resulting image revealed a pattern of dots. Each dot is a different size, proportional to the blackness of the original photograph, in that location.

The resulting negative, called a "half-tone" was exposed onto a metal plate coated with light sensitive emulsion called a resist. Development etched the metal. The etching pattern replicated the dot pattern.

Ink was applied to this plat. It was used much like a rubber stamp to make ink copies of the original. Just look at any picture in a newspaper or your school book. Use a magnifying glass, you will see this dot pattern. Digital photo editing software can apply a dot pattern image to your digital images.