Histogram
A graph of how many pixels in an image fall at each brightness level, from black on the left to white on the right.
In depth
The histogram shows distribution, not correctness — there is no such thing as a good histogram shape, only one that matches the scene. A snowscape should pile up on the right, a night scene on the left. What matters is whether data is stacked against either wall, which indicates clipping, and whether the tones you care about sit where you want them.
Examples in practice
A correctly exposed low-key portrait shows a tall spike near black and a small hump at midtones — alarming-looking, but exactly right.