Note 3. ISO International Organization for Standards

Please refer back to 1, where I briefly touched on ISO.
ISO refers to the setting which controls the sensitivity of the light which acts on the sensor.
ISO settings can go up to 51,200 (in some high end cameras) and to put it in context when film was used the sensitivity was anything from 25 ASA to 400 ASA (slow film to fast film) and available in black and white, colour or colour reversal (slide film).
Sometimes certain films could be 'pushed' during processing.

Now remembering that without sufficient sensitivity, along with appropriate shutter speed and appropriate aperture your image would be either under exposed or at worse nothing recorded; if the sensitivity of the film or sensor was too low. And of course if your aperture and shutter were not appropriate.

With the range of sensitivity on modern digital cameras it is possible to meet most if not all high or low light conditions without resorting to Flash.
A word, you do not need low light to use Flash, Flash along with ambient light can be quite effective.

It is all down to what you like and what you envisage the image to turn out like. In other words it DEPENDS.

Like film 'high' ASA comes at a price. It shows more prominent the grain in the film and in digital that, what was grain is called 'noise'. So high ISO can bring along with it higher noise which could, depending on the effect you want, be obtrusive. But as technology advances there are software packages to counter this.
That is another subject on itself within the heading of 'POST PRODUCTION'

To recap then ISO is another setting to consider along with aperture and shutter speed but needs to be understood to be manipulated. Generally the 'auto' setting will take care of everything, almost but it will not do your thinking for you.
Go back to 1. where I spoke of 'your subject' how do you want it to turn out?
Then the camera setting will be it DEPENDS?

What should I do next you might say?
Familiarise your self with your camera settings, see how they alter the image when you change one or all of the settings.
When you are pretty conversant with them. Take plenty of photographs and learn from them.
If someone thing is not quite clear after that find out what it is and look at different magazine articles where an in depth summary might be just what might help.
Find out what subjects you really want to photograph.
Learn your craft by either taking plenty of photographs, looking at other photographers work. Try and work out how they recorded what they did and what did they want you to 'see'.
Then find your own style by looking at what does the image tell you.
Does it record the image or does it go further and tells a story?