Exposure Value Johnny,
Almost all digital cameras have an Exposure Value (EV) Compensation setting. This setting is needed because the camera can sometimes make incorrect assumptions about the lighting of a photo. Changing the EV will make sure your photos are always correctly exposed.
Your camera is calibrated to expose images correctly for scenes that have a mix of dark and light areas. This works well in most situations because our images normally tend to have brighter areas (sky) and darker areas (shadows). On Auto, the camera chooses a brightness setting around mid way between the brightest and darkest areas and makes sure that area is correctly exposed. This causes problems, however, when all of your image is very bright (like a white sandy beach or snow), or very dark (like deep green forest scenes). For very bright scenes, like at the snow, the camera thinks the very bright part should be the ‘mid point’. This causes the snow to appear gray (because it’s mid way between white and black). Similarly for darker images.
Just lifted that from the net to try & explain it