Contrast is the comparison between light and dark, strong and weak, and so on.
In the case of images, it refers to a "difference in lightness" of light areas and dark areas in an image. As contrast increases, light areas get lighter, and dark areas get darker, thereby making the difference in lightness clearer. On the other hand, as contrast decreases, the difference between light areas and dark areas gets smaller, thereby making the image blurry.
