Extremes in light can trick a camera meter to improperly expose a photo. Bracketing overrides the exposure settings of a camera. You bracket by taking a series of shots of the same scene at lighter and darker exposures.
Many digital cameras can be set to bracket automatically. When auto exposure bracketing is enabled, the camera takes a short series of consecutive pictures of the same scene. Exposure Values automatically change in plus and minus incremental steps.
Some digital cameras not only have a bracket setting, but can automatically merge the bracketed images in-camera for better dynamic range.
The first three shots, on the left, were taken with the source of light coming from behind the subject. This is known as back-lighting.
Without changing the Exposure Value from the default exposure reading, back-lighting causes the main subject to be under exposed, ie. dark.
The next three shots, on the right, were taken when the main source of light fell on the front of the subject. The strongest light came from behind the person taking the picture.
Without adjusting the Exposure Value when light falls too strong on a subject, the image becomes overexposed. What results is an photo with washed out, blown out areas.
Better to under than overexpose
It is generally better to under- rather than overexpose an photo. When editing, it is often possible to pull out detail from the darker, underexposed areas. When portions of an image are washed out due to overexposure, there remains little or no detail to pull out in the blown-out area.