I always use the following analogy to explain ISO. See if it makes more sense to you.
The project is to use marbles to form a picture on an empty tray. But you get to decide the size of the marbles you want to use.
Restriction is:
Low ISO = smaller marbles
High ISO = larger marbles
Using Low ISO, you will require more marbles to form the picture. It will take longer but the image will be clearer (because more marbles = greater attention to details)
Using High ISO, you will require less marbles to form the picture. It will take a shorter time but the image will not be as clear (because the number of marbles used is less).
In the above example, you can replace marbles with "light".
Similarly on the camera, the higher the ISO, the less light it will need to capture the shot and the lower the ISO, the more light it will need. How much light you give it will depend on your shutter speed and aperture.
So... best advice = take a few low ISO and a few high ISO pictures and see the difference. Play around with low ISO, slow shutter (or wide aperture) and high ISO, faster shutter (or narrower aperture) and see the difference.
Most importantly, have fun