Doesn't it feel great when you buy a camera, actually understand some two main functions of shooting, then you come come across three letters in all caps lock...
Our eyes are constantly changing the amount of light that enters our eyes because light situations change constantly. We don't actually realize how dark it gets outside until for instance you have take your camera out, change some aperture, change some shutter speed, but with an ISO left at 100 you are like "Dang! It's dark outside"...
Oh shoot, you probably don't understand that reference because you still don't know what ISO is, sorry here goes... ISO is light sensitivity. That's the gist, ISO has a range from 100 (zero grain/noise) up to 6400 (lots of grain/noise). Now depending not he camera you have this range may top out from 4000-24,000. But the idea is that from an ISO at 100 this means this is no noise and grain, noise and grain is when you see lots of course dots on your image, like in black and white films. It looks like static on an image. What ISO does, is it fabricates surrounding lights, and makes the light around you (in your composition) more bright. Now this is where the scale comes in to play, the closer it is to 100, the less grain you get on an image, but that also means the less sensitive it is to the light. The more you move your ISO up, say 800 on up, the more sensitive to light as well as more grain you get. An example being...
When I shoot wedding receptions, it is more then likely dark, I use flash to brighten up the scene or shot. Now, using a flash takes batteries (that I don't want to constantly change) so a way to counteract that is to make my flash seem brighter then it actually is. So, depending on my light scenario, I can set my flash to a low power to reserve battery life, as well as jacking up my ISO to make my flash way brighter, by doing this, I'm saving battery power but also using my tools to make my scene appear brighter. Now, granted this comes with some consequences like more noise or grain, but in the end you are the photographer, you make the calls. Besmear about it, don't just jack up ISO because you feel like an ISO of 100 is getting all the glory.
Back in the good ole film days, you would have to buy film, the film would come in most cases ISO 100 or ISO 400, depending on what you liked to shoot your light sensitivity would seem normal, or brighter. I could give a rule of thumb, but here's the thing, all shooters are different, all light scenarios are different. But what you need to know is ISO is there to be used when it's needed, and in so many cases I use it!!
For images and a chat about it, check out my video on ISO in the "Topher's Classroom" menu.