By the definition of ISO is a measure of the level of the camera sensor sensitivity to light. The higher the ISO setting we are the more sensitive the sensor to light.
To get a clear picture of the ISO setting on our camera (ASA in the case of photographic film), just think about a community of bees. An ISO is a worker bee. If my camera set at ISO 100, it means I have 100 worker bees. And if my camera set at ISO 200 which means I have 200 worker bees.
The task of each worker bees is collecting light entering through the lens of a camera and make pictures. If we use a lens aperture of identical and equally we set at f/3.5, but I set my ISO at 200 while your 100 (think again of worker bees), then the picture who would have finished faster?
Broadly speaking, when we increase the ISO setting from 100 to 200 (the aperture is always constant – we lock the aperture at f/3.5 or through Aperture Priority modes – A or Av), we shorten the time required in the manufacture of a photo on the camera sensor we get to half (2kali faster), shutter speed of 1 / 125 to 1 / 250 sec. As we add more ISO to 400, we cut the time of making photographs until the other half: 1 / 500 sec. Every time Exposure shorten as much as half, we call for 1stop exposure raise.
You can try this sense in the case of the aperture, we try to set the shutter speed is always constant at 1 / 125 (or via the Shutter Priority mode – S or Tv), and change-change your ISO setting in multiples of 2; missal from 100 to 200 to 400 … and so on, look at changes in the amount of your aperture.