ISO
 

ISO is simply a camera setting that will brighten or darken a photo. It sets the camera sensor's sensitivity to light. 

As you increase your ISO number, your photos will grow progressively brighter. For that reason, ISO can help you:
 

1. Capture images in darker environments
    

2. Be more flexible about your aperture and shutter speed settings.
    

However, raising your ISO has consequences:
 

1. A photo taken at too high of an ISO will show a lot of grain, also known as noise, and might not be usable. 
    

2. You should only raise your ISO when you are unable to brighten the photo via shutter speed or aperture adjustments.