Occasionally you will produce better photographs if you override the exposure settings suggested by the camera. You can use exposure compensation, which allows you to increase or decrease the exposure set by the camera. Up to two stop compensation can be applied.
Exposure compensation works by altering the shutter speed or the aperture (sometimes both), depending on the shooting mode set on the camera.
The amount of compensation needed is something you will have to learn by experience.
Auto Exposure Lock (AEL)
Your camera normally meters the exposure for each shot. Sometimes, though, you want to lock the exposure, so that it does not change as you move the camera around.
When shooting a portrait, for example, move in close to meter from just the face. Lock the exposure. Now move back to include the background, perhaps composing with the face off-centre. As long as * or AEL on your LCD screen remains visible, the original meter reading will be used, exposing just for the face.
Refer to your camera manual to discover how to set AEL on your camera.