When too much light reaches the film or sensor, so the picture looks too bright. This can happen if the shutter speed is too slow, the aperture is too wide or the ISO is too high for the light. Detail in the bright areas is lost and they become pure white, or blown out.
Some photographers overexpose on purpose for a light, airy look. Sometimes extra exposure is needed to correct the meter. In snow scenes, for example, the camera's meter often makes the snow look gray, so photographers add exposure to keep it white.
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