ISO adjusts the sensitivity of your camera to light. As in all photography there are compromises. As you increase the ISO to make your camera more sensitive, there is a slight loss in quality (noise). The higher you go the more you loose.
But modern cameras can work at high ISO values not imaginable only a few years ago. Modern cameras can create very clean images at even 1600 ISO and higher.
I would rather have a image that has slight noise but is sharp, rather than an image that had camera shake, just because I didn’t want to put the ISO up and loose some quality.
Noise in an image can usually be dealt with in Postproduction, a unsharp image is much more difficult to save.
Next up Aperature….