Plugins are intended to add custom functionality into the application, that doesn't fit the general needs and is more specialized for a sub set of users. These plugins can easily be created by creating a specific C# class library.
A plugin template to get a starting point and also a detailed description how to work with the template can be found in the Plugin Template Repository
Furthermore plugins can be made officially available in the plugins tab by submitting a plugin manifest into the main Plugin Manifest Repository. This repository will automatically update the remote host with the available plugins to show them inside the application to download. All details how to create a manifest and how to submit it, can be found in the README of the repository.