The Tet Offensive was meant to catch the US off guard and to drive the US out of Vietnam. This offensive was thought to drive the US out because of all the anti-war protests in the United States. This offensive was going to be carried out by fighting on two fronts, by doing this the Vietnamese thought they could spread the US military out so much they would fold causing them to give up one of the cities they had captured. The Tet Offensive was viewed as a turning point in the war. The offensive was said to cause the president to realize that he would need to send more men and more resources over if they were going to completely defeat the Vietnamese. Since there had been many anti-war protests in America this was not really possible.
More info: http://countrystudies.us/vietnam/29.htm