Print on demand (POD) is a printing technology in which book copies do not print until the company (bookshop) receives an order for this exact book. It allows printing singular or small quantities. Books are printed for a fixed cost per copy, regardless of the size of the order. While the unit price of each physical copy is greater than with offset printing, the average cost is lower for very small print jobs.
In other words, your book is only printed when the reader orders it and – very importantly – will pay for the print. The author does not bear any printing or shipment costs.