A dummy is a rough model of how a book will look.  The artwork is just sketched in and the text is usually gibberish that’s designed to take up the same amount of space as the final text.  (Sometimes creative people write very funny things in the text boxes of dummies, since they know only their co-workers will see it.) 
The whole idea of a dummy is to give everybody a chance to notice any major goofs in the way the book is designed… before a lot of time is spent on finalizing it.