what is ISO? 

 ISO measures the sensitivity of the image sensor. lower the iso less sensitivity your camera is and the opposite higher the iso more sensitivity your camera is going to be.

How ISO works technically (in camera) & visually (in photo)? 

how the iso works technically is the lens of your camera is trying to let light in but the body of the camera isnt so thats why the photos are blurry. For how it works visually is if you have a lower iso the photo is gonna be more clear not blur to the picture, but if you have a really high iso like a 6400 the picture will have  good amount of blur to it. 

What happens to the exposure triangle when you change your iso?

so when we kept changing the iso to make it increase on the light meter we had to change the shutter speed to make it go faster due to your f stop had to the stay the same so you have to change the shutter and make the shutter like 1/250 a really fast shutter.