On a ship, steerage is the lower level where cargo is stored or where passengers with the very cheapest tickets are accommodated.
Most senses of this noun are pretty old-fashioned today. It used to be fairly common to crowd third-class passengers into a single steerage hold for an ocean voyage, and many immigrants coming to the U.S. in the 19th and early 20th centuries traveled this way. These days steerage is only considered appropriate for luggage and goods, not humans. Another meaning, "the act of steering a ship," is mainly used in literary contexts.