ISO

                               f/4.0
                               1.3
                               ISO 100  

                                                         

f/4.0
0.5
ISO 200

f/4.0
1/5
ISO 400

f/4.0
1/8
ISO 800

f/4.0
1/15
ISO 1600

f/4.0
1/30
ISO 3200

f/4.0
1/60
ISO 6400

What is ISO?

It's the level of sensitivity of your camera to light. 

How ISO works both technically (in camera) and visually (in photo).

In the camera it's the sensitivity level that is has towards light. The lower the ISO number, the less sensitive it is. The higher ISO, increases the sensitivity. Visually, it gets pictures to look with more or less exposure.

The relationship to the exposure triangle.

It helps to get more definition in your pictures, with the light to make it darker or lighter depending on the effect you want to bring out in the picture.