It's a self playing piano. It's what the invitation note from Rupert to John mentions in the beginning: "None of us have your talent, of course, but I have recently acquired something to keep you interested if you decide to come. It's not perfect, but sometimes when you close your eyes, it's like someone else is in the house...
I'll make sure to have it running by the time you arrive. Don't be fooled — there are no ghosts in Galley House!"
I'm guessing John is a good piano player - also explains his interest in it.