The KDE SVN is an online archive that stores the source code for KDE. It is possible for anyone to browse the SVN by visiting the web SVN. The name "SVN" comes from the software that the archive uses, which is called Subversion.
The archive contains not only the version of KDE that is under development but also the current stable release and the history of changes to KDE.
If someone refers to a new feature as being "in the SVN" it usually means that it is not available yet but that a working version has been created and will probably be available in the next release.