Traffic sign words are important for drivers to understand and obey in order to ensure the safety of everyone on the road. The four words that make up the term 'traffic sign word' are 'Thru, ' 'SLOW, ' 'MERGE, ' and 'STOP.' 'Thru' is a signal to drivers that they can continue on their current path without interruption. 'SLOW' is a warning to drivers to reduce their speed. 'MERGE' is an instruction to drivers to join with another lane of traffic. Finally, 'STOP' is a command to drivers to come to a complete halt.