A histogram is a graph that displays the distribution of tonal values in an image. It represents the brightness values of the pixels in the image, with the darkest tones on the left and the lightest tones on the right. 

The vertical axis of the histogram represents the number of pixels at each brightness value, while the horizontal axis represents the range of brightness values. 

A histogram is a useful tool for photographers because it can help to identify problems with exposure, such as overexposed or underexposed areas of the image. A well-exposed image will typically have a histogram with a smooth distribution of tonal values, while an image with clipping or blocked-up shadows or highlights may show spikes or gaps in the histogram. 

Histograms can be accessed and viewed in many digital cameras, and are also available in most image editing software. By examining the histogram of an image, photographers can make adjustments to exposure, contrast, and other settings WHILE TAKING THE IMAGES to ensure that their images are properly exposed and have the desired tonal range.