The Shelf app is part of a mission to help users access and share their own activity data. After installing, you're prompted to connect to services like Netflix (for watching activity), Goodreads (for reading activity), Steam (for game play activity), Apple Music and Spotify (for listening activity).
Shelf currently makes this personal media consumption activity available for you to share (importantly, users have control over what gets shared and especially what gets highlighted).
This can help you as a Goodreads user (or Apple Music, or Spotify) share your activity with friends who do not necessarily use the same services.
At present, Shelf does not allow re-export of your data. It aggregates data from other primary sources, so ideally those primary sources themselves would give you access to your data.
The source code (markdown) for this article can be found on GitHub.