A digital sourcebook is a completely digital version of one of the published books, such as the Player's Handbook or adventures like Curse of Strahd.
You will get the book re-created in digital format, as well as unlock all of that book's content for use throughout the toolset - both for current tools and anything on the roadmap (such as encounter building/ combat tracking, etc.).
Here is a preview page from the Curse of Strahd to illustrate:
