With modern cameras exposure metering is built into the camera. Light falling on the camera sensor is measured and translated into a recommended shutter value (in Aperture Priority) or a recommended aperture (in Shutter Priority).
Cameras can only cope with a limited amount of contrast in a scene (areas of dark and of light), much less than the human eye. So the metering system is forced to choose between providing some detail in either the shadow or highlight areas. Changing the metering mode can help you give priority to certain areas of the scene in order to gain the desired exposure.
Camera have a couple of metering modes which can cause some confusion. They are:-
Manufacturers give different names to this mode but basically the meter measures from various spots around the image and works out an average reading. This option is the the best for the majority of your shots under average lighting conditions. I use it virtually always.
As the name implies, in this mode priority is given to the centre of an image even though the whole image is still measured. This mode is useful where you want to bring out detail in the central area, e.g. a person/people in a brightly lit scene where they would normally be in silhouette.
This mode is used to take a measurement from a very small area within the image. It is used where the subject is the most important area and nothing else is important, e.g. a bird in dark surroundings or a bright object such as the moon in a dark sky.
As explained, all metering modes provide recommended values which can be overridden using the EV Compensation button/dial. This is used to change the recommended exposure. If an image is too dark (underexposed) dial in a +ve EV value. If an image is too bright (overexposed) dial in a -ve value. It is also used when your subject is darker/lighter than the background.
I hope this helps.