An embedded topic is a topic that has its embed_url property set to the URL of an external site. I am only aware of this being done when topics are published to Discourse via the API. For example, the Discourse WordPress plugin and the Discourse javascript embed code both create embedded topics.
If you are publishing your topics to Discourse from an external site, this approach would make sense. You won’t be able to use this approach for topics that are created directly on Discourse though.