As the project organizer, you have permission to set specific settings on any chapter in your project. Here are the settings options for chapters:
Normal: This is the default mode for all chapters. Every member of your project can see the chapter and add their stories and pictures to it.
Locked: A locked chapter is still visible to everyone, but only you can add content to it. This can be handy if you have a chapter with specific instructions that you don't want people inadvertently adding their stories to, for example.
Hidden: Chapters that are hidden are ONLY visible to you. The rest of the members of your project will be unable to see the chapter or the content in it. This can be very handy if the book you're making includes a surprise that you want to keep secret. For example, if you're making a baby announcement in the book that you only want revealed when the printed book is opened, a hidden chapter is the way to go.