By default most cameras will take an average light reading of the whole picture. This can lead to your subject being incorrectly exposed due to the brightness of the background. In this example you can see that the girls face is under exposed (too dark).

 

Using Exposure Compensation, Exposure Lock or Spot metering it is possible to adjust the exposure balance to correctly expose your subject.
In this example the girls face has now been correctly exposed giving a much more pleasing result.
 
Another method is to use fill-flash to lighten the subject.
Conversely, if the background is too dark your subject can be "bleached out".
Using Exposure Compensation, Exposure Lock or Spot metering can also correct for this.