Canon Rebel T2i Quick Start Manual page 61

The Histogram
The histogram is used for judging the overall contrast of
the image. Values increase in luminosity from left to
right: shadows on the left, highlights on the right.
Here are three different exposures of the same subject:
The top histogram indicates a very bright image,
possibly overexposed.
The middle histogram indicates a high contrast image,
meaning that it has values more evenly distributed
throughout the range of luminance. This is a correct
exposure.
The bottom histogram indicates an image that is too
dark, possibly underexposed.