ISO is used in digital photography to set the sensitivity of the sensor. What it comes down to is when you are taking a picture, the more light you have, the less sensitive you need your ISO. Inversely holds the same. If you are taking an image at night, dusk or in places with less light, you will want to increase the sensitivity for the digital sensor that captures the image.
This image was captured on the day when I had used a high ISO. But I pulled the grain really hard to get outlines of this beautiful barn to show. Doing so created a very grainy background, but I like it just the way it is. Imperfect! Like me! 🙂
This image was taken with a high ISO. I had my ISO high when I was taking pictures of the moon at night, which is amazing, if you can push your camera far enough, but does not fair well when using in high sun. I forgot to change my settings and this is what happened.
This same photo I was able to salvage, into what I think is pretty darn cool, by using adjustments in Lightroom. Adobe Lightroom was pretty intimidating at first and I still don’t use all of the functions, but it is an amazing work space.
Most digital cameras have auto modes that will adjust your ISO, to the settings you are working in. This is perfectly fine, but as you establish a style of images that you want to produce, you can learn how to control all of your settings, which will get you better quality images, just the way you want it!