That means that you can manipulate the slope of the ITF to emphasize features in the image. For example, in this image there is one large peak in the histogram at the low end of the pixel range, and a pair of smaller peaks at the midrange. A little tinkering with the image reveals that the large peak represents the darker features of the crabs' face and the background, while the two midrange peaks represent brighter highlights in the sandy beach and the mans' shirt. To bring out details in the crab, we can increase the slope of the ITF over the large peak in the histogram, at the expense of flattening it out over the midrange and high end. Equalization fits the curve of the ITF so that the intensity for each pixel value is proportional to the percentage of the histogram that is below that value.
All pixel values have a contrast that is proportional to their contribution to the image.