What is ISO?

https://photographylife.com/what-is-iso-in-photography

ISO is one of the three essential components of photography, the other two being shutter speed and aperture. ISO is the level of sensitivity of your camera to light. A higher ISO level means higher sensitivity to light, and vice versa. Having a higher sensitivity means you can take pictures in places with not a lot of light, but it also means that your picture will come out with more "grain"/noise. Having a low ISO will give you high quality pictures. The ISO number has an effect on the speed it takes to capture an image. For example, ISO 100 will take a picture in 1 second, while ISO 3200 will take a picture in 1\32 of a second. Low ISO should be used when there is plenty of light, and high ISO should be used when there is little light and you want to quickly capture an image.