ISO is the digital sensor or film sensitivity to light. The higher the ISO, the more sensitive it is to light.
When increasing the ISO, a digital camera amplifies the light that hits the digital sensor, which will allow you to use faster shutter speeds, but causes “digital noise”, a random speckling / dust effect on the image. The higher the ISO, the more digital noise.
The lower the ISO, the camera does not need to amplify the light, so there will be much less digital noise.