This is the ability to discriminate between the bright and dark areas of whatever you are looking at. A healthy retina can help discriminate very small differences between light and dark areas, and this helps give richness of detail such as facial features, patterns on clothing, and road signs. When you turn the level of contrast up on your TV or computer monitor, you are darkening the dark areas and lightening the light areas, and in the extremes the fine details (the grays) are washed out.
Eye conditions such as cataracts and macular degeneration as well as retinal disease caused by conditions such as diabetes can reduce contrast sensitivity. In these cases more light is necessary for daily activities such as reading or sometimes even just walking around.