ISO is the sensitivity of the camera sensor, essentially acting as an amplifier to raise the exposure(brightness) of the photo digitally

When abjusting the exposure, we balance the three settings of the aperture, shutter speed and the ISO.

HOWEVER, when ISO is increased too much, it introduces digital noise. These are inaccurate colour representation being prominant in the photo due to the amplication of electric signal noise.

Therefore, it is best practice to set the aperture and shutter speed first before setting the ISO if needed.