Short answer: these are remnants of snapshots that were deleted but their parts are still being used by other parts of the system..
So, you had a snapshot, and somewhere down the line you created a bookmark. That bookmark required some of the data that is in that snapshot. Then, you deleted the snapshot. The system threw away all the data in the snapshot that wasn't being used by the bookmark, but it has to keep that "shared" space so the bookmark will still function. That extra stuff is kept in the "Held Space (was 'former-snapshot-name')" To get rid of this, you'd need to remove some bookmarks, and then the held space will automatically be removed.