Contrast
The gap between the lightest and darkest parts, which makes shapes easy or hard to read.
See it in Dither engine. Under Image processing, raise Contrast and watch the dots separate.
In plain words
Contrast is the difference between the light and dark parts of a picture. High contrast makes shapes stand out, with deep shadows and bright highlights. Low contrast looks flat, like a view through fog. Raising contrast pushes dark tones darker and light tones lighter. For text, strong contrast between the letters and the background makes reading easier.