The combination of ISO, aperture, and shutter speed determines the exposure of a photograph.
ISO is used to indicate how sensitive a camera’s image sensor is to available light.
Aperture how wide the hole is in the lens that lets light through.
Shutter speed tells us for how long the light will be allowed to pass through the lens.
One way of looking at the three components of exposure is this:
ISO. A higher ISO equals increasingly more grain, or noise, in your result. Some photographers add grain on purpose for effect, particularly when shooting in B&W.
In a dark scene, or at night, increasing the ISO from 100 to 3200 will ensure you get the photograph, but will result in a grainy image.
Aperture. The aperture setting is expressed in f stops, f2, f4, .. f16, etc. The higher the number, the smaller the hole the light passes through. One result of that is depth-of-field. Depth-of-field also depends upon how far your subject is from the camera. A graphic explains things better:
We call the combination of ISO, aperture and shutter The Exposure Triangle. Basically, to get the correct exposure we balance all three. So, when one changes the others must change also to achieve the overall balance again.