What is ISO?
In very basic terms, ISO is simply a camera setting that will brighten or darken a photo. As you increase your ISO number, your photos will grow progressively brighter. For that reason, ISO is a good tool to help you capture images in dark environments or be more flexible about your aperture and shutter speed settings.
However, raising your ISO has consequences. A photo taken at too high of an ISO will show a lot of grain, also known as noise , and might not be usable. So, brightening a photo via ISO is always a trade-off. You should only raise your ISO when you are unable to brighten the photo via shutter speed or aperture instead (for example, if using a longer shutter speed would cause your subject to be blurry).