The histogram is a type of graph which represents the tones in a scene and it reveals exposure information. It looks a bit like a mountain range, with its varied shapes of peaks and valleys; and these peaks and valleys provide you with exposure information.
A correctly exposed scene is indicated by a fairly even distribution of the peaks. If the scene is underexposed, or darker, the peaks will bunch over to the left side of the graph. If the the scene is overexposed, or lighter, the histogram peaks will bunch over on the right side.
Clipping, below, refers to the data (white peaks and valleys) touching the very outer edges of the graph.