What is ISO on a digital slr and why do i need to change it?

I'm a noob. I have a nikon d3100, and i know how to change it and everything about it except what it is. I think it might be for movement speed? I don't know, help please!

Here you go, this will help you understand: http://www.google.com.au/...+explained

ISO is an acronym for International Standards Organization and in the case of cameras is a measure of how much light is needed to properly expose a picture. If you are shooting outdoors on a bright, sunny day, you can use a "slow" speed, say about ISO 100 since there's an abundance of light available. If you are shooting night scenes then you need to use a "faster" ISO setting, perhaps 400, 800 or above. These settings increase the sensitivity of the camera so that it can take a properly exposed picture in dark situations. The trade off is in fine detail - the faster the ISO setting, the less detail your picture will have and vice versa. This is only usually a worry to professionals or if you are going to make large prints from your photos. For most snapshot situations, the amateur will not notice the difference unless you use rather extremely high or low ISO settings. Experiment with the various ISO speeds for special effects and choose what pleases you.

Here's a hint, the brighter the day or light is, the lower the ISO you use. The faster the subject is moving also matters too. But generally you would be alright leaving you camera on 200 or 400.

ISO was the system back in the film era where they measured the density of the particles that reacted to light (silver Nitrate I think it is). Anyway, the more dense, the lower the ISO. Same basic concept can be applied to a digital camera

On lower ISO's (400 and below) you will not see that much noise and will get a crisper shot, the downside is that you need more light because there are more "particles"

on higher ISO's (800+), depending on what camera you have, here is where you will most likely start to see some noise in your picture. On the plus side, however, you don't need as much light.

so, the darker the setting, the higher the ISO, generally
the Lighter the setting, the lower the ISO, generally.

if you are outside in daylight, go down to 200 or less ISO for crisper shots
if you are inside, use a higher ISO to get a fast enough shutter speed so you do not need to risk having a blurred shot.

It is how sensitive the sensor is to light. The lower the number the less sensitive it is. The higher the number the more sensitive it is. Changing it will affect the cameras exposure. Keep in mind that the higher the iso the more noise in the image. You should use the lowest iso you can for the amount of light you have. There's a really good book you should read called "understanding exposure" by Bryan Peterson. He does a great job of explaining how to adjust shutter, aperture, and iso to get the effect you want in the photo.

You could leave it on auto until you have an understanding of how it works.