Lighting is key in photography, one main factors is ISO. If you find your self in a very dark situation using the lowest aperture and shutter speed possible. Then you might want to bump up your ISO. ISO in simple digitally brings up the brightness in your picture.. and thats about it. The big problem though is when you put it too high, and Grain/Noise starts creeping in to your image. If your only posting images your images on social media and not anything too high res, then 9 times out of 10 you could get away with it. If your a person looking for crystal clear images being blown up on to billboards, screens etc. Then ISO is something definitely to deal with. Professionals photographers often use software such as photoshop, to clean up there image and remove grain.
Above are two images, one taken at ISO 100 and one at ISO 3200. The difference is pretty easy to see. ISO can range from 100 to 25600 in some cases.