Tonal curves on a photograph are often skewed in some direction. Perhaps the entire image is too light or too dark, perhaps the light tones are not light enough or they are too dark. It could be that the same thing has happened in the shadows because of exposure or lighting. Or frequently the entire middle tones are shifted in one direction or another (Gamma). It is relatively easy to correct this but when these tonalities are significantly stretched, gaps can appear in the areas affected and this visually translates to posterizing or banding. The more ‘slices’ of information we have to perform this correction in programs like Photoshop or Lightroom, the better. Jpegs are saved in 8 bit nomenclature (256 tonal ranges) but all higher end cameras have the ability to save in RAW format which approximates 16 bit image files. This has tens of thousands of ‘slices' of tonal range.

Above is an exaggerated example of where this happens when a compressed highlight (represented by far left histogram) gets ‘stretched out over a greater range, As expected, note how the 8 bit image on the far right separates into segregated tonalities. If this represented a light sky area with clouds and the sun, there would exist significant banding in the 8 bit Jpeg image that would be very obvious, whereas in the central 16 bit modification (of the far left histogram) there is no posterization.

This is just an example. If this actually was a histogram of some light area the whites would be 'clipped'. Depending on the photograph, this could be very visually intusive.