Subversion is a source control system like Visual SourceSafe. It keeps track of all versions of a set of source files.
Eric Sink has written an excellent overview of basic source control concepts. You can find it at http://software.ericsink.com/scm/source_control.html .
A Subversion repository is a collection of all files related to a particular project. You can think of it as an instance of a Subversion database. The FlexWiki repository lives at flexwiki.svn.sourceforge.net, in the folder /flexwiki.
If you have only ever used Visual SourceSafe in its default configuration for source control, you should be aware that Subversion works quite differently. SourceSafe operates under a "lock-edit-checkin" model, wherein only a developer "checks out" a file, preventing anyone else from working on it, then edits it and ultimately performs a "check in", releasing their lock and allowing the next person to edit the file.
Subversion (like most other source control systems) does not default to this behavior. Rather, it uses a "edit-merge-commit" model. In this model, developers retrieve files from the repository and edit them without locking them. When it comes time to save changes back to the repository, Subversion will check to see if anyone else has made changes to the files you changed while you were working on them. This is called a "conflict", and in many cases Subversion can handle it automatically. If not, the developer can use any one of a number of tools to reconcile any differences and store the merged files back into the repository. This model allows multiple developers to continue working on FlexWiki without anyone blocking anyone else's work.
Some of the terms that Subversion uses may be unfamiliar to those who have not worked with Subversion or similar source control systems before.
The usual sequence of events when working with Subversion is:
Repeating steps 2 through 4 as often as necessary.