Understanding Iso

F/3.5 

Shutter speed 0.6"

Iso 100

This picture came out more grainy compared to the highest of 3200.

F/3.5

Shutter Speed 1/25"

Iso 3200

This picture came out more speckled compared to iso 100.

Di) 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.

Dii) Works technically by allowing the light being more or less sensitive to light. If you were low iso you would have a low aperture and a lower shutter speed . If you were high Iso, you would need a higher aperture and a faster shutter speed. Works visually because it picks up light easily so you would need a narrow aperture. If it doesn't pick up light easily you will need a wide aperture. 

Diii) Iso is the light sensitivity of your camera, the lower iso the less sensitive, the higher iso the more sensitive it is, so if you have a high iso, you would need a higher aperture and a faster shutter speed. If you were a low iso, you would need a lower aperture and a slow shutter speed. That how it relates to the exposure triangle.