This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
It is used to identity a library within a page. When you use a tag, you want to tell from which library it comes from. Imagine a scenario where you have two tags with the same name, but from from two different libraries. You need something to tell which tag is from which library, and that's what the prefix is for.