SVN - Subversion. SVN is a version-control system that allows multiple developers to maintain a single codebase. It is intended to be a replacement of CVS.
In the context of Ogre, there is an anonymous SVN server that you can download the latest source code of OgreAddons from. Many times, there are improvements and patches in SVN that are not in the latest release. The current link to this SVN repository can be found on this page: Ogre Addons guide
The Ogre core code is now hosted on bitbucket. It's best to access it via Mercurial. See all the inforamtion on this page: Ogre Mercurial guide.