This setting allows you the most control, and you can set your own shutter speed and aperture value using the dial. For Powershot camera, press the up button, choose shutter speed or aperture number, then turn the dial to set a value. For the Rebel, turn the dial to set the shutter speed, or hold down the Av button while turning the dial to set the aperture number. You can check the meter to make sure it's balanced, so your values will create a correct exposure.
If the meter points toward the left (underexposed), choose a slower shutter speed or a smaller aperture number (which opens the aperture to let more light in). Or if the meter points toward the right (overexposed), choose a faster shutter speed or larger aperture number (will close the aperture to reduce the amount of light let into the camera).