Sometimes the readings measured by your EOS may not give the exposure you would like.
If you are photographing a snow-covered landscape or bride in white, for example, your camera's exposure meter may be fooled by the quantity of white within the frame. In this case, you need to increase the exposure compared to what the camera recommends.
The exposure compensation feature on your EOS lets you do this simply. This is found either on the LCD panel on top of the camera, or on the screen on the back of the camera. The scale usually runs from -3 to +3, with a small index mark showing the selected value. On many EOS cameras, this is done by pressing the '±' button on the back of the camera and turning the main input dial moves the index mark from '0' to give a plus or minus exposure compensation setting. Check your camera's Instruction Manual for full details.
A bright snow-covered landscape might need up to +2 exposure compensation. A setting of +1 will probably be adequate for the bride in a white dress. Minus exposure compensation is used for dark-toned subjects, especially if they will fill the frame or are against dark backgrounds - blue and purple flowers against green foliage, for example.
With Live View mode, you can apply exposure compensation and see the result before you shoot.