Managed scripts are shell scripts which are managed centrally by an administrator and can be referenced as a build step within jobs.
The scripts are managed by the Config File Provider Plugin, go to the "Manage Jenkins" screen and click on the "Managed files" item.
After installing the "Managed Scripts" plugin, you have a new option to create a managed script in the type list.
Now you get to the screen to create your script. You're able to give it a name and a small description. In addition, you have the option to define a list of arguments you want to inform the users about. These arguments will be shown to the user as a hint and within the script you're able to reference these as if you would pass in arguments on the command line ($1, $2,...). Every environment variable from within a build will be visible to the script, too.
Within the job, a user is able to select the required script from a drop-down menu. The defined parameters are shown in respect to the selected script. The link enables the user to have a read-only look at the script.