Bright subject, dark background
If the subject in the frame is brightly lit (ea. by direct sunlight) and the background is dark, the subject will be overexposed in normal AE photography.
Exposure compensation
To prevent this, move up close to the subject and press the shutter half way down to lock the AE mechanism. Then move back to frame and shoot the picture you want.
Use the AE Lock mechanism to control exposure in special lighting situations.
If the subject is in front of a bright background or fight source, the foreground will be underexposed. To prevent this, aim the camera towards the ground (widening the aperture by I or 2 steps) and press the shutter release halfway down.
The camera will take a reading for the foreground and will lock at that reading as long as the shutter is held down half way. Then aim at camera at the subject and press the shutter all the way down to get a correctly exposed picture.