ISO specifies how much light your chip gets to reach maximal values. For film, this was fixed and depended upon the film. Digital fotochips can work on different levels of light, but less light means more noise. Modern cameras are getting more and more tolerant though and work very well even at high settings. The best quality is still at min ISO, though.
Shutter speed specifies how long the picture is taken. A picture that takes 1 second to take will show nothing but movement blurr if you're photographing a running athlete. A mountain on the other hand doesnt care if you only require 1/1000 sec to take the picture, or 30 seconds.
The third value is lens aperture. If the lens is wide open, a lot of light will be on the chip, but the area of sharpness will be small. This is actually desireable in many situations, such as portraits. If the lens is closed, less light will get through, but the area of sharpness will get much wider.
Depending upon what you want, and how much light you have to work with, different settings for these three values might be desireable. In some cases you might even want a gray filter to reduce the available light, so you can make movement visible.