A supposed ‘perfect histogram’ is an even mountain. However this is rarely the case.

A histogram is a representation of the pixels exposed in your image in a graph. The left side of the graph represents the blacks and shadows, the right side represents the highlights and bright areas and the middle section is mid-tones (middle or 18% grey) this is effective to use a grey card. How high the peaks reach represent the number of pixels in that particular tone.screen-shot-2016-10-21-at-12-04-28