As mentioned above, you occasionally may need to step in and adjust exposure. This may be to compensate for any metering errors caused by the camera. It can also be for effect, to create a particular look or mood. Adjusting exposure is achieved with the exposure compensation function of your camera. (With the exception of Manual exposure.) This is achieved either by turning a dial, or by altering a menu option.

Exposure compensation is adjusted in fractions of a stop, either 1/3 or 1/2, typically between plus or minus three stops. When you want to underexpose an image (or correct overexposure) you apply negative compensation, when you want to overexpose (or correct underexposure you apply positive compensation.
Exposure compensation is a very powerful tool to refine exposure. However, most cameras ‘remember’ how exposure compensation is set. If you do not reset exposure compensation you may find that your subsequent shots are incorrectly exposed. Setting exposure compensation back to 0 once it is no longer needed is a good habit to get into!