"Macchia" is an Italian word for "spot" or "stain" but essentially it means a quick oil sketch capturing in a few strokes the basic and accurate structure as well as impact of some scene or objects. This is made possible by limiting the sketch to at most 4-values and the artist's palette to a limited number of colors organized into a color pool. The macchia paintings will have a strong (some say "extreme") chiaroscuro (light vs. dark) which makes for a bold contrast between the background and the foreground. The pictorial space is reduced to at most seven basic shapes. There is an attention to the paintings surface and texture.
Stronger paintings have "impact" or "force" (the "walk in test": is the painting noticed when you walk into a room, especially from a distance and do people linger to spend more time there?) Macchia paintings often begin in a monchrome color (just darks and lights in one color) and the artist does not go to the next stage until a pleasing harmony of lights and darks in achieved. This is a compositional strategy based on reducing the overall picture into a limited number of large shapes and values.
The Italian word "macchia" has multiple meanings but as "pattern" it refers to the underlying pattern of lights and darks. Seeing "macchia" requires training the eyes and the mind to recognize interesting and harmonious abstract patterns. Both Leonardo and Michelangelo spent time studying the "macchia" stains on walls to obtain inspiration for figurative compositions.
Although strong/light dark contrasts often have more impact, as the work of Titian and other old masters demonstrate, complex and multi-shape paintings with subtle value changes are also beautiful and compelling, so it it hard to make a general rule that the macchia must always conform to a simple-minded concept of strong darks and lights.
Additional Information on the relationship between "macchia" and the Risorgimento period