What is the private note, and who reads it?

It goes only to the host and the moderators they invited, and never appears in the public guestbook.

The private note is for the thing you would say to the host directly rather than in front of everybody. It never appears in the public guestbook, is not included in the printed guestbook keepsake as a public message, and is not shown to other guests.

It is read by the host and by any moderators they have invited to help — private from other guests, not private from the host’s team.

You can leave a private note on its own, without a public message, if that is all you want to say.