A plug-in is a file or a set of files that adds new features or capabilities to the Stingray editor. You can customize the behavior of the editor by choosing which of these plug-ins you want to load into the editor at any time.
For the mechanics of selecting what plug-ins you have loaded, see Add and remove plug-ins using the Plugin Manager.
Some of the standard, built-in features of the editor are actually implemented as plug-ins under the hood, to keep the features modular and self-contained -- and to make sure we can handle the taste of our own dog food. In addition to these standard plug-ins, which you get by default when you install Stingray, you can easily load other plug-ins written and distributed by third parties. (But since this plug-in system is just getting started, we don't have any third-party ones to share with you -- yet.)
You can also write your own plug-ins that customize the editor for your own usage, or for distribution to other Stingray users in your organization or in the larger community. See the Stingray SDK Help for details.