ISO
ISO is a camera setting that will brighten or darken a photo. As you increase the 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 setting.
Shutter speed
Shutter speed is exactly what it sounds like, 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.
Aperture
Aperture refers to the opening of a lens's diaphragm through which light passes. Lower f/stops give more exposure because they represent the larger apertures, while the higher f/stops give less exposure because they represent smaller aperture.