ISO is a system of measuring the sensibility of the film. I took three photos, all the same but with different ISOs.
Exposure Time: 1 / 50
Exposure Time: 1 / 100
Exposure Time: 1 /125
As you can see, the picture gets brighter as the ISOs go higher. The aperture gets bigger, leaving more light to get in. The time the film is exposed goes down, because less light is needed. Since the aperture is very big, exposing the film too long would bring too much light, so the film is exposed less, giving us the right amount of light.