<p>I am developing a website using CodeIgniter and PHP.  Some of my friends suggest I start using Subversion to take care of my organization.  Could someone give me a basic explanation of what SVN is without the jargon and also if it is practical for a framework where 5 files are needed to run a single page?</p>

SVN is a
version control
system - which means that it stores various previous versions of a given file, and allows many people to work on the same file, and in the end put all the changes together. It also helps you go back to a previous version if necessary.
You should use that in your project if there are a lot of people working on the project. But even if you are the only one working, it will be easier to manage the project if it is on version control.