SVN Up is a simple module that, as the name suggests, updates a particular folder of your Drupal site from an SVN code base.
Shell access to your server and subversion installed. Your apache or www-data user must also have the ability to store their SSH credentials.
This module should never be used in production. When enabled, it has the ability to add/update/alter the files on your site.
Where it can be handy is in a staging location, where you may wish to check out custom themes, or modules that are being maintained in a subversion repository. This module gives you a one click interface to fire svn up from the web.
Please review the included README.txt for detailed instructions on installation and configuration.