In photography, ISO is how sensitive a camera is to the amount of available light. A higher ISO number means that it is more sensitive to light.
Cameras have a sensor that gathers light and transforms it into an image. The base ISO in a camera is the lowest ISO number of the sensor that can create the highest image quality.
ISO numbers range from 100 to 200. Each step between numbers doubles the sensitivity of the sensor. If there is a lot of light, the lowest ISO should be used to have the highest quality image. A higher ISO should be used when there is not a lot of light.