"Exposure compensation" is the act of deliberately altering exposure from the value suggested by the camera to make pictures brighter or darker. Although in P, S, and A modes, the camera automatically adjusts exposure, the value it selects may not always produce the results you intend. Everyone has her own preferences when it comes to exposure, and you may find that a brighter or darker image is closer to what you intend. If this is the case, use exposure compensation. Digital cameras let you view pictures as you take them, so view the results try again if they're not to your liking, adjusting exposure upwards or downwards for brighter or darker pictures.
1: –0.3 EV
2: +2 EV