Bluelines (or the “approval proof”) are the pre-publication proof that a printer sends a publisher to look over one last time before the publisher gives the printer the “green light” to start printing the book. This is your very last chance to catch any mistakes in the printer’s set-up, etc. Only one or maybe two sets of bluelines exist for each book.
As shown in my post about It by Stephen King, these days most bluelines are just a digitally printed stack of loose pages, but one of our new printers actually delivered a recent approval proof as a bound (with tape) book, which was a nice surprise. You can see a little bit of the DJ bluelines under the book, too: