"Rhino" is the nickname for the back-end service component in the Ambra stack. (The Ambra stack's "Rhino" has no connection with the JavaScript engine of the same name.)
Rhino is a loosely RESTful service that exposes a JSON-based API. Articles are ingested into it by HTTP requests. Then, it serves both the article content and metadata to other services such as Wombat.
See the Ambra Project documentation for an overview of the stack and user instructions. If you have any questions or comments, please email dev@ambraproject.org, open a GitHub issue, or submit a pull request.