A histogram is a graphical representation of the distribution of data. In video cameras, histograms usually represent the tonal or color distribution within an image. The horizontal axis represents tones; dark to light from left to right. The vertical axis represents the amount of pixels included in that tonal value.
A histogram is a visual and data based representation of the exposure of an image that is used to aid the DP. If the pixels are grouped toward the left, the image may be under exposed. If the pixels are grouped toward the right, the image may be blown out.
Histograms can also represent the distribution of individual colors within the image, which is helpful in color correction or color balance.