Ok, lets start from the beginning.
What is the ISO?
ISO is the sensitivity of the camera sensor.
Its just indicate:
the higher is the value, the higher is the sensitivity,
and lower the value, the lower sensitivity.
Proper combination of:
1. ISO and 2 other parts of exposure triangle (2. Aperture and 3. Shutter Speed) give us a possibility to have a perfect image. Besides full automatic camera mode, all cameras have 3 very useful shutting modes:
Photographer should decide about aperture value, and camera will select proper shutter speed, or
2. Shutter Priority
Photographer should decide about shutter speed value, and camera will select the appropriate aperture value.
In both cases (aperture and shutter priority) a photographer must decide about the ISO value, depending of the light conditions, which literally means if you are shutting in the bright conditions (day light), photographer will choose lower ISO.
Lower ISO practically means that you will have less noise on the image, which indicates better image quality.
If you set camera ISO on the higher position, (taking pictures in the lower light situation), photographer must set higher ISO value, which literally means that the image will have some noise on the image.
After explaining the basic of exposure, we are able to jump on AUTO ISO, which is our goal in this article.
The most desirable camera shutting mode in the situation of unpredictable lighting situations is (3) MANUAL shutting mode with AUTO ISO. In order to explain better these conditions, imagine situation that sport photographer have to capture players on the field which is one lighting area, and and the audience which is second light area. In described conditions a photographer must set ISO value every time when press shutter button, depending he/she is shutting players, audience or someone else.
How to set AUTO ISO?
The majority of cameras today have auto ISO possibility, my camera is produced 6 years ago and have it. You have to find ISO Sensitivity in your camera menu, check auto ISO sensitivity and switch it to ON. The optimal ISO sensitivity according to my opinion is 6400, which will minimize possible noise on your image, and minimum shutter speed of 1/125 seconds, which is also by my opinion very safe value.
Make experiment with different shutting modes
If you are for example shutting a model who is entering the house and walking right to the living room, and if you are using Shutter Priority Mode you will not have control over your shutter speed.
It may happen that for final result you will have blur images since your model is moving. Or if you are in the Aperture Mode, there is a possibility that your image will be underexposed, depending of the the lens you are using and lighting conditions in the house.
Manual Mode + AUTO ISO
On the other hand, if you are using Manual Mode with AUTO ISO, you will have total control over aperture and shutter speed and the camera will decide the proper ISO for you.
If you set minimum shutter speed at 1/125 which is enough to freeze the motion and maximum ISO at 6400, you will have a perfect set up by my opinion.
The only issue you may have, since you are not controlling ISO (up to 6400) is to have little noise on your images. But on the other hand imagine that instead of little noise you have blur image, this is something that you can not fix.
Also, if you have full frame camera, you will have very good images even on ISO value of 6400, since they are presenting good results in low light environment.
Pick Your Cam