Day 4 in 31 Days to a Better Photo is about learning what ISO is and how to apply it to your photos and more importantly how shutter speed and ISO are used in combination to balance the amount of light that is in the camera.
For a full explanation please visit the link to the my3boybarians website to hear what Darcy has to say about ISO and shutter speed.
The "assignment" was to take pictures in Program mode and notice how changing the ISO would also change the shutter speed. I practiced on my cup of coffee this morning and you will notice that even as I increase the ISO, the light in each picture is almost identical. The aperture changes a bit with the pictures because it wasn't set and the camera was "guessing" at what I wanted, so pay no mind to that.