Contrast is the visual difference between two elements — in size, color, weight, shape, or spacing — and it's one of the most direct ways a designer can tell someone "look here first." Unlike Gestalt grouping, which is about relationships, contrast is about difference: the eye is drawn, almost involuntarily, to whatever breaks the pattern around it. A single red button in a sea of gray text isn't noticed because it's well designed — it's noticed because it's different from everything near it.