When we manually expose our photograph, we've got those three points: ISO, aperture, and shutter speed. Let's just talk about the ISO. ISO is sensitivity of light onto your camera's image sensor. The higher the number, the more sensitive your camera is to light, the more you can take a photograph, or the faster you can take a photograph in a lower light situation. The problem with that is, and this is where you've got to be very, very careful, is that your camera is two things.
It is stupid, and it's lazy. If you're on any form of automatic setting, the first thing your camera is going to change is the ISO, and it's generally not going to work out okay. It will take the ISO up nice and high so you can take a photograph, say at night, and as a result you're going to get very, very, very grainy quality in your photograph. If you have a look at this photograph of Las Vegas, all the blacks are beautifully black, and all the color is nice and vibrant.
That's because I kept the ISO nice and small, 100, 200. Even 400 is fairly safe. Anything higher than really 800, and I'd start seeing that noise and that graininess coming through the photograph. By using our ISO effectively, we can make sure that our photographs are always the highest quality possible. That's how we use ISO as part of our exposure triangle.