Subversion is an open source application used for revision control. It is sometimes abbreviated to
SVN in reference to the name of its command line interface. Subversion is designed specifically to be a modern replacement for CVS and shares a number of the same key developers.
The Joomla! project uses an SVN respository to store it's code and make it publicaly available. Each time a change is made to the code (a bug is marked as [FIXED]) it is committed to the SVN. If u want to test the fix that has been committed there are 2 ways to get the new code :
1. U can checkout the SVN on your local system
2. U can download a nightly build (*)
More information can be found here :
http://dev.joomla.org/content/view/17/60/(*) Recommended for non-experienced users.