Aperture
This lets the photographer control the amount of light reaching the film or image sensor. The aperture is changed along with the  shutter speed to control the amount of light that enters the camera.  This amount of light is called the exposure.  Typically, a fast shutter speed will not let much light in (meaning the picture could come out too dark), so the photographer will use a larger aperture to correct for this.  A slow shutter speed will need a smaller aperture to stop the picture coming out too bright.