Aperture can be defined as the opening in a lens through which light passes to enter the camera. It is an easy concept to understand if you just think about how your eyes work. As you move between bright and dark environments, the iris in your eyes either expands or shrinks, controlling the size of your pupil.
Shutter speed is the length of time the camera shutter is open, exposing light onto the camera sensor. Essentially, it is how long your camera spends taking a photo. This has a few important effects on how your images will appear.
In basic terms, 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.