What is ISO?
The ISO setting is very important in photography. If you are wondering what, exactly, ISO means...here is an absolutely useless definition. ISO stands for International Standard of Organization. What?! OK, so the definition is useless. Here is what it means to us as photographers. The ISO means the camera sensors sensitivity to light.
Most all cameras have a native ISO setting of 100. This is the optimal setting for achieving a image free from digital noise. ISO numbers will then rise from 200, 400, 800, 1600, 3200, 6400 etc, with some variations in between depending on the camera manufacturers settings. Quite simply, the lower the ISO (closest to 100) the less sensitive the camera sensor is to light, while the higher the ISO (going towards the cameras highest ISO number), the more sensitive the camera sensor is to light.
What does this mean in practical terms? In situations where there is a lot of bright light, you will be able to shoot at a lower ISO, thereby getting the highest quality image. However, in other situations, you may start to see a darker image, at which point you will need to do one of several things. You could raise your ISO, thereby making your sensor more sensitive to the light, but risking image quality. You could also slow your shutter speed down to allow more light to reach the sensor. You could use a more open aperture setting (f 2.8, for example). ISO, Aperture, and Shutter Speed are what is known as the Exposure Triangle. Changing one of these settings affects the others. Take a nighttime sport like football. Football is a fast paced game, and the photographer will want to freeze the action so the photos are not blurry. In bright daylight, you may be able to use ISO 100, but will need a shutter speed of about 1/800th and an aperture of f 2.8. That is why at the professional levels you will see these huge lenses on monopods. Those lenses are usually something like a 300 mm, f 2.8 lens. That will allow you to let the most light into the sensor, while still having a fast shutter speed.
But what happens when you shoot a sport like football at night, when light levels are low from poorly lit stadiums? From personal experience, I often shoot at ISO 6400, Aperture f 2.8, shutter speed 1/800th to 1/1000th. I have to raise my ISO that high to get a properly exposed image that is not too dark. What happens if my ISO is lower? The image is too dark. Lowering the shutter speed makes the photo blurry, and raising the aperture makes the image too dark. So, as you can see, they are interrelated. The problem with an ISO as high as 6400 is that most consumer level cameras either do not go that high, or if they do, the image is noisy (grainy). Some of my students use cameras that only go to ISO 1600, and have lenses that only open up to f 3.5. In this case, it may very well be impossible to freeze the action of a sport. In situations like this, I recommend trying to take photos of other game situations (huddles, reactions of players and coaches, formations, etc) as they will be easier to get a quality image.
For more information on shooting indoor/poorly lit sports, see this article.
Figuring out your Exposure Triangle may seem daunting, but do not stress over it. With some practice and attention to settings, you will figure it out quickly and it will become second nature.