ISO is the third and often the most under-estimated of the three exposure parameters. It is not a part of the exposure value parameter. ISO refers to the sensitivity of the sensor to light. It is measured in whole numbers. The lowest is usually 100 (though some cameras can shoot at even ISO 50), the point of least sensitivity on most cameras.
The highest has no limit (theoretically) as the boundary is being pushed with each new camera. In a technical sense, the sensor’s sensitivity does not change. What’s important is what happens after the camera captures an image.
The software inside the camera will take an image and boost the light signals to produce a well-exposed photo even though the light is not commensurate. The disadvantage in this is the noise (or static) is also boosted. You would probably see that as specs of white in the final image. It is best to keep ISO at the lowest number possible.