Your question should be what ROOTing is and why would one do that to the Samsung Galaxy S. ROOTing a device like the smartphone Samsung Galaxy S, means that one get a type of access to the default settings and applications installed. This means that one can remove them or change them as they wish, but as expected, this action should only be taken by those who know what they are doing, and at own risk. This action can, if done something wrong, end up with your phone not functioning anymore.
Before proceeding you need to read some guides, understand the instructions, and download some tools. This page has a list of the tools you need for this task, instructions for both the PC and for the Mac as well as instructions for un-ROOTing, so that you can remove the phone form the computer and use it as a phone. The title of the article mentions that even the Vibrant and the Captivate models will work to be ROOTed with the method and tools used there.
There are people who are more comfortable with following visual instructions, and just for them youtube is hosting some videos with this purpose. One such video can be seen here and it seems that this method is for the international model, the i9000. If you want to use it for any other model, feel free to do so, however if it will ruin your phone, you will be the one to be blamed.
Do you need to be reminded not to do this if your phone is still under its warranty? Or about performing a backup of the files and other data you have on the phone so that you make sure you don't lose something important? Plus, you should read more tutorials if these weren't enough, just you stay safe without phone.