The refresh link actually runs "garbage collection" on the database, in that it cleans up what is referenced in the database from the albums directory.
SVN code is the subversion repository. It is a repository of development code, not necessarily stable. SVN allows you to "check-out" code from the development branch of the core code, which in effect is the "cutting edge" code that is being worked on. In fast paced development, this code can sometimes change quite frequently, and SVN allows a method to have a complete up-to-date version of the development.
For zenphoto, this means you can download the latest development code as it stands now, which is version 441.
The SVN repository is at http://www.zenphoto.org/svn, and you can look at a number of clients (for Windows, TortoiseSVN) to get started getting the latest code.
If you have trouble (or if you don't want to bother trying the SVN client), let me know and I can always create you a export .zip file of the SVN that you can grab if you want to try the dev code.