PPB stands for paper, printing, and binding. It is a dollar amount. It is also called the unit cost.
PPB is the cost only of the physical book: how much money (per book) we will have to spend on the paper, the printing, and the binding process.
A PPB for an average-sized, 32-page picture book, printing in four-color, and with a print run of more than 10,000 copies will often be about $1. As soon as you want spot UV or deboss or a funny size or the page count starts running up, you're adding on to that cost. If you want pop-ups or a sound chip or some hoo-ha like that, those will really cost you.
Things not included in the PPB:
The cost of shipping the books from China to the US.
The cost of shipping the books from the dock to the publisher's warehouse.
The cost of shipping the books from the publisher's warehouse to stores.
The cost of running a publisher's office.
The editor's salary.
The designer's salary.
The production manager's salary.
The marketer's salary.
The publicist's salary.
The many sales staff's salaries.
The costs incurred in the process of editing (eg, a fact-checker, a proof-reader, a copyeditor)
The costs incurred in the process of design (eg, buying new fonts).
The costs incurred in the process of marketing and publicity (eg, making displays, paying for advertisements).
The cost of free books to send to reviewers, etc.
...And several other things I've forgotten. Wondering where all the money between your PPB and your retail price goes? It's not into the publisher's pockets.
Authors and illustrators don't make a lot of profit on books, but neither do publishers.
There are some industries where a 50% profit margin is expected. Ha-ha! In publishing, we're aiming for around 10%.