SVN, or Subversion, is a source control system intended to allow a team or group to work on the same files and projects simultaneously, and to be able to revert file and project states back to previous versions.
Once projects are placed on the SVN repository, they can be checked out (imported) by anyone with access to that repository. SVN repository connections allow you to import projects from your repository to your workspace, which you can make and upload changes to.
SVN functionality can be accessed from the SVN Repository Exploring Perspective, accessed from Window | Open Perspective | Other | SVN Repository Exploring.
See the Subversive User Guide for more information on SVN.