Engines can be considered miniature applications that provide functionality to their host applications. A Rails application is actually just a "supercharged" engine, with the Rails::Application class inheriting a lot of its behavior from Rails::Engine.
Engines are the reusable rails applications/plugins. It works like a Gem. Famous engines are Device, Spree gems which can be integrated with rails applications easily.
rails plugin new [engine name] --mountable
|--mountable||option tells the generator that you want to create a "mountable" and namespace-isolated engine|
|--full||option tells the generator that you want to create an engine, including a skeleton structure|
Engines are very good options for creating reusable plugin for rails application