Exposure compensation lets you modify the exposure up or down from the metered reading by a specified amount. By doing this, you can continues shooting using the modified meter reading setting and get good exposures.

But, there may be time when you want to shoot with more than the exposure compensation available on your camera. In those cases, check to see what exposure the camera is setting, then choose the manual shooting mode with an exposure more or less than the camera meter system recommends. Check your histogram to confirm the exposure.
Adapted from Top 100 Simplified Tips and Tricks, Digital Photography 3rd Edition (Wiley, 2007, $20)_