Understand that every single photograph is a combination of ISO, Aperture and Shutter-Speed.
Every. Single. One.
ISO refers to sensitivity of the film or sensor.
Aperture refers to how much light is hitting our film or digital sensor.
Shutter-Speed is how long the film or sensor is being exposed to the light.
So, let's talk about sensitivity.
A low ISO (25-100) is perfect for bright conditions. It is not very sensitive at all and will need a lot of light. As such, a roll of ISO 100 film is referred to as a "daylight" film.
ISO 400 and up has an increased sensitivity.
It is perfect for low-light conditions and if shooting during the day, the increased sensitivity means we can use it for "sports/action" so we can freeze our subject in place.
Unfortunately, as we increase sensitivity, we also increase digital noise or film grain, depending which format we used.
Many, like myself, prefer the look of film grain because under the right conditions, it can add a nice artistic touch to our photograph. While with digital noise, it can be so overwhelmingly bad, that it can look like garbage, to be honest.
With film, we pop the roll in our camera body and that ISO becomes the starting point for determining what our exposure will be.
One of the biggest advantages to digital cameras is that we can change the ISO from one shot to the next. With film, once you put in a roll of ISO 100 film, that is the speed you shot at until it was all used up.
Since a low ISO has a significantly lower grain/noise footprint, it is usually a go-to ISO for photographers.