SVN stands for SubVersioN.
An SVN is a place for nerds to make programming code available to more nerds. They can then do as they wish with the code, including use it to upload newer versions (revisions of the code). This cooperation between both small and large communities allows for things such as server software, utilities, tools, plugins, games, and even projects as large as operating systems.
SVN's are also more commonly known as a repository for open-source projects.
The http daemon just got updated, so I'm going to go download the new SVN repository and update my version of httpd.