Exposure triangle. 

Shutter speed

Aperture is were if you have a bigger aperture the more light is going in to the camera witch makes the photo more brighter .

The speed at which the shutter of the camera closes. A fast shutter speed creates a shorter exposure — the amount of light the camera takes in —while a slow shutter speed gives the photographer a longer exposure. 

ISO is simply a camera setting that will brighten or darken a photo. As you increase your ISO number, your photos will grow progressively brighter. For that reason, ISO can help you capture images in darker environments, or be more flexible about your aperture and shutter speed settings.