ISO is one of the three pillars of photography, along with Shutter Speed and Aperture. ISO is the sensitivity of your sensor. We usually increase the ISO in low light situations such as indoors or during dusk and dawn. When your sensor is more sensitive, less light is required to enter the camera to create a well exposed image.
There are two main aspects controlled by ISO, which are visual noise and exposure.
Noise is how grainy the image is. The higher the ISO, the more grainy it is. A low ISO results in smoother photos. Ideally, keep your ISO low and only increase it when there is less light in the environment.
ISO also affects exposure;
The higher the ISO, the brighter the image.
The lower the ISO, the darker the image.
Below, you can see how changing the ISO affects the exposure.
I hope this tip on ISO will help you get more creative with your photography.