More than a board
A washboard is a wooden plank with a ribbed surface. Its legs are placed in the water-filled laundry tub. The item of clothing is placed on the ribs of the plank and is scrubbed with a brush (or a corn cob) and a bar of soap.
Washboards were used all across the world for a long time, as can often be seen on old paintings, from Italy to Japan. In countries where washing machines are not common, they are still popular. In some Southeast Asian countries for example, such as the Philippines and Indonesia, clothes are still often cleaned with washboards.
Nowadays, the ribs are often made of steel. The Surinamese washboards (they are called wasuma in Sranantongo, the lingua franca of Suriname), were made of tropical hardwood that hardened the more it came into contact with water. You would go to the washboard maker on the market, and to a different craftsman for the laundry tub.