"Ghost Plots" is the name of new information by J.K. Rowling which can be unlocked on Pottermore.
The new information can be found in "Hagrid Arrives". It is in zoom layer 2: when the paper on the right side of the door is clicked, the information is unlocked.
J.K. Rowling's thoughts
|“|| This is a personal expression, which has nothing to do with tales of the dead.
Over the seventeen years that I planned and wrote the seven Harry Potter books (not to mention Quidditch through the Ages, Fantastic Beasts and Where to Find Them and The Tales of Beedle the Bard), I generated a mass of information about the magical world that never appeared in the books. I liked knowing these things (which was fortunate, given that I couldn't stop my imagination spewing it all out) and often, when I needed a throwaway detail, I had it ready because of the background I had developed.
I also found myself developing storylines for secondary (or even tertiary) characters that were superfluous to requirements. More of a wrench were the plots I worked out for some much more important characters that had to be sacrificed for the bigger story. All of these I inwardly termed 'ghost plots', my private expression for all the untold stories that sometimes seemed quite as real to me as the 'final cut'. I have occasionally been in conversation with a reader and made mention of part of a ghost plot; looks of consternation cross their faces as, for a split second, they ask themselves whether they have accidentally skipped twenty pages somewhere. I apologise to anyone I might have accidentally wrong-footed in this way; the problem is, literally, all in my head.