Shutter speed refers to the amount of time that light is allowed to fall on your camera’s sensor when taking a photo. It, in conjunction with the aperture and ISO setting, determines the exposure of the photo. Given a consistent aperture and ISO setting a fast shutter speed will let in less light and a slow shutter speed more light.
In addition a fast shutter speed will freeze motion, capturing a particular moment, while a slow shutter speed will result in a blur, capturing the motion. Slow shutter speeds can lead to blurred photos due to camera shake when hand held.
In automatic mode shutter speed is controlled by the camera.