Yes, thanks for sharing it!
"Overhead" is kind of a catch-all for the publisher's day-to-day costs of being in business as a publisher--costs that unlike plate costs, or buying paper, or royalties, are not attributable to a specific book.
So it would include salaries, building maintenance, purchase of equipment, and so on. It's usually added into the "profit and loss" calculations as a standard percentage, such as 35%.