This plugin adds the ability to directly execute Groovy code.
To configure available Groovy installation on your system, go to Hudson configuration page, find section 'Groovy' and fill the form as shown bellow.
To create Groovy-based project, add new free-style project and select "Execute Groovy script" in the Build section, select previously configured Groovy installation and then type your command, or specify your script file name. In the second case path taken is relatively from the project workspace directory.
The plugin also adds the functionality of the Script Console to the project configuration page. You can schedule your system management script...
...and then observe progress in the build log.
The plain "Groovy Script" is run in a forked JVM, on the slave where the build is run. It's the basically the same as running the "groovy" command and pass in the script.
The system groovy script, OTOH, runs inside the Hudson master's JVM. Thus it will have access to all the internal objects of Hudson, so you can use this to alter the state of Hudson.