To understand exposure, it can be helpful to think of your camera as a tool for gathering light. Exposure is simply the amount of light we allow in. If we allow too little light in then the picture will be dark, or underexposed. Too much and it may become overly bright, or overexposed.
Ambient light bounces off objects and surfaces in a scene and your lens channels it through the camera and onto the sensor – this is what creates the image. In Auto or Scene mode, Canon cameras are very good at determining the optimum amount of light for a balanced exposure, but we can have greater creative freedom if we take control ourselves.