A timmerman is a carpenter. In the days before all houses were made of concrete or brick, carpenters built the frames. In the Middle Ages, most houses were made of wood. The houses had timber frames filled up with wicker and mud/clay. Later, bricks were used to fill in the timbers.
Many carpenters were also employed to build ships. In the 17th century, the Russian Czar Peter the Great came to Zaandam to be apprenticed as a shipwright.