Radiographic contrast is the density difference between neighboring regions on a plain radiograph.

High radiographic contrast is observed in radiographs where density differences are notably distinguished (black to white).

Low radiographic contrast is seen on radiographic images where

adjacent regions have a low-density difference (black to grey). 
Contrast

A low-contrast image will show little difference in density between structures of interest

A high-contrast image will show a larger difference in density between structures.