In very basic terms, ISO is the level of sensitivity of your camera to available light. The lower the ISO number, the less sensitive it is to the light, while a higher ISO number increases the sensitivity of your camera.

ISO-exposure

As you can see with these 4 images, because the ISO has been changed the light has increase but swell as the light you also can see that the graininess has increase as the images move to the right.

999

5555

684984

3218

5724