Based on the definition, ISO is a measurement of sensor sensitivity level to light. The higher the setting of ISO, the more sensitive the sensor to light is.
To get a clear explanation about the ISO setting on your camera (ASA in the case film photography), you can imagine a group of bees. An ISO is a worker bee. If your camera is set on ISO 100, it means that you have 100 worker bees. And if you set the ISO into ISO 200, it also means that you have 200 worker bees.
Each worker bee has a job to collect light entered into the lens of the camera and create a picture. If you use identical lens and the aperture of each is set to e f/3.5, but the ISO is set on 200 meanwhile the other is set on 100 then imagine which picture will be finished faster?
The bottom line is when you add the setting of ISO from 100 to 200 (on constant aperture – you lock the aperture on f/3.5 or through mode Aperture Priority – A or Av), you make shorter the time needed to create a photo on camera sensor almost a half (2 times faster), from shutter speed 1/125 to 1/250 seconds.
When you add the ISO to be 400, it means you have cut the time up to the half more: 1/500 seconds. Each time you make shorter the time to exposure up to a half, try to set the shutter speed constantly on 1/125 (or by shutter priority mode – S or Tv) and change the ISO setting in multiples of two, such as 100 to 200 to 400 and so forth. Look at the change of your aperture. Good luck!