If you owned a film camera, you may remember going into the camera shop and having a bunch of different film speeds to choose from. Most of the time people stuck with 400, or turned the package over to find out which one was right for the conditions they'd be shooting in. There was 100 speed film for "sunny days", 200 for outdoors/cloudy, 400 indoor with flash, and 800 was for "low light". Digital cameras retained the idea of film speed in a setting called "ISO".
Either way, "film speed" or "ISO" is a measurement of how fast the film (or your digital camera's sensor) will record the light that makes up your photo. The faster it records, the higher the number, and the better it'll perform in low-light situations. -Have you ever tried taking photos inside and they came out blurry? If you would have used a faster speed film or a higher ISO, you'd get less of that blur because the camera would have been able to record the image faster.
Of course, you don't get something for nothing. There's a tradeoff for choosing a higher ISO, and that's image quality. With film, you get graininess. With digital, you get noise (those grainy, multi-colored specks that you sometimes see in digital photos). There are programs out there, like Noise Ninja or Neat Image that can help reduce the noise in your photos, but generally it's best to keep the ISO as low as you can for the conditions you are in.
(Below is an example of a noisy image:)
So, if you see that your low-light photos are coming out blurry on your camera, try raising the ISO number to the lowest number that gets you clear, unblurred photos. (Then remember to put it back to 100 when you're done so you don't end up taking photos in daylight at ISO 800 and get unnecessarily noisy photos as a result).
This is one of the 3 things you can adjust when using your camera in Manual Mode. So, if you are wanting to learn Manual Mode, this is an important concept to understand. -With ISO, a bigger number means more light can be recorded in a given amount of time.
**NOTE** Film speed and ISO are not exactly the same, but they are close enough that for our purposes and, especially in this intro, you can treat them as if they are 1 to 1.
If you don't understand something or have trouble with any of my tips, feel free to contact me via a comment on this (or any) article, or see my website for my email address and I'll be happy to help you out!