It is a tool used by photographers to direct the attention of the viewers' to the subject. There are two types of contrast namely tonal contrast and color contrast. They differ in the tones from the white to gray to black.
Contrast is the scale of difference between black and white in your images. Without contrast you wouldn't have an image because there wouldn't be any differentiation between light and dark; everything would be black, white, or a single shade of grey somewhere in between.
