A Bookmark is esentially a savegame feature in the game. Whenever you feel it would be good to insert a virtual bookmark into your book, you only need to tap the green bookmark icon at the bottom left of the screen. You will need to create an account first, as bookmarks are saved into the cloud. If you change your device for example, you will only have to login with the same account on your new phone/tablet and you can go to any of your bookmarks.
To access your bookmarks, tap “Menu” at the bottom right corner and tap “Jump to Bookmarks”. If you are logged in, you’ll see the list of all your bookmarks.