Each pixel of the image have a range for how dark/bright it can be before it is absolutely white or absolutely black. When you take your shot with the camera some parts of the image might be overexposed (absolute white) or underexposed (absolute black).
The camera tries to make sure that your subject does not become over or underexposed. This is done with the cameras light metering. The camera meters how bright or dark the subject is and then decides how bright or dark the image should be captured. Sometimes the metering is wrong or, does not apply to your particular motive, when that is the case you can avoid a badly exposed image either by changing your metering settings, you should be able to tell the camera to meter the entire image or at the focus point, or at the focus point and somewhat around it ;). You can also use the exposure setting to tell the camera to adjust the image a few steps darker or brighter.
Later on when your in your computer the image hopefully have a correct exposure, if not you can adjust the image to become brighter or darker, however your overexposed (complete white) or underexposed (complete black) areas can not be "fixed" and will look abit patchy. There is however nothing wrong with these areas as long as they don't contain any details that is lost due to their bad exposure.