To quote from Wikipedia: "Offset printing is a commonly used printing technique where the inked image is transferred (or "offset") from a plate to a rubber blanket, then to the printing surface." The plate making process is costly. But once you create a plate, you can use it to generate a large number of copies. Therefore, printing just a few copies is not cost-effective with offset printing, but the cost per copy decreases with increase in the number of copies printed.