Shutter-Speed  is the easiest property for people to grasp because it can have such a dramatic visual  impact on a photograph, like that of a waterfall.

Inside of a camera, there is a light-proof curtain (shutter-blades), that opens and closes. When it opens, it, exposes the film or digital sensor to light.

If something moves, from the time the shutter-blade opens, until the time it closes again, it will blur.

If the Shutter-Speed is set fast enough, it will freeze the subject in motion.

In this example, the Shutter-Speed runs the range from 1/100 of a second (fast), down to 0.4 seconds (long).

Notice how the water, the only thing which is moving, changes in appearance from one photograph to the next.

In photographing waterfalls, I find that 1/2 of a second to 2 seconds (or so) can often deliver a very pleasing photograph.

So after setting the ISO, I may very well consider setting my Shutter-Speed value next...