Similar to a traditional creative brief, a content brief is a document used to list requirements for a piece of content that is going to be created. It's typically created by an editor to be sent to a writer, or created by the writer themselves to be reviewed by someone else before writing the full article.
Content Briefs can be used for all types of content, from landing pages to articles - basically any content you can create. Content Harmony is focused on helping you build briefs more effectively, since creating them manually takes most teams hours.