The word "monochrome" literally means "one color." Therefore, a monochrome image only includes one color, but may contain many shades. In computing, "monochrome" typically refers to a two-tone image, rather than one with several shades of a single color. For example, a monochrome monitor uses one color for the background and another to display text or images on the screen.
Before color monitors became standard, most computers had monochrome displays. These displays often had a black background with green text, though some displayed text in other colors, such as red or orange. While this may seem like a rudimentary way to display text, it was sufficient for typing documents, since computers still offered more text editing capabilities that a typewriter. Even after color monitors became the norm in the 1980s, monochrome displays were still used for several years as computer terminals.
Today, monochrome computer monitors are rare. By the time LCD displays replaced CRTs monitors in the early 2000s, monochrome screens had already been obsolete for several years. Now, even basic terminal displays support a wide range of colors. While you might not see monochrome monitors today, monochrome displays can still be found in other electronics, such as watches, timers, and digital clocks.
NOTE: Monochrome is not the same thing as grayscale. A grayscale image is a type of monochrome image that only contains shades of gray. Additionally, monochrome and black-and-white are two different things. The phrase "black-and-white" may refer to a monochrome image that only includes the colors black and white or a grayscale image with multiple shades of gray.