The shutter is a curtain that's always closed. When you click the shutter button, the curtain opens for a bit to let the light in, and then closes again. You can control how long it stays open. The longer it's open, the brighter your image.Take note: both the aperture and shutter control how much light comes in. The only difference is the aperture can change its size, and the shutter can change the duration it's open.