Contrast is the difference in brightness between parts of an image. The contrast filter makes bright pixels brighter and dark pixels darker if contrast is increased or vice versa if contrast is reduced. The contrast filter controls allow you to adjust the contrast between -50 (reduced contrast) and +50 (increased contrast).
Images require proper contrast and brightness for best viewing. When contrast is too high, light parts of the image are too light and dark parts are too dark. When contrast is too low, light and dark parts of the image appear to be shades of grey.
Contrast enhancement yields images that may appear sharper without the need for slow sharpening filters.