The shutter speed, aperture size, and ISO are the three factors of the exposure triangle
There is double the exposure between ISO settings. so ISO 200 is twice as sensitive as ISO 100.
The higher the ISO, the more light is amplified. The higher the ISO the more prominent is the amount of noise that is picked up by the camera.
To reduce noise in high ISO pictures you can set the camera to reduce noise. Also, multiple programs and software can help smooth over pictures such as Photoshop and light room.
Chromatic noise is the speckling in pictures that are multicolored while Luminance is when the camera struggles to watch determine the brightness of a pixel and results in the image appearing to have multiple black dots.
Changing ISO is needed usually when you are in darker areas that you would like to take a picture of such as at night. Another example is when you need a quick snapshot in a dimly lit area.