Tet was an offensive launched by the communists against urban South Vietnamese and American targets in 1968. It’s major significance lay in the impact it had on US public opinion, strengthening opposition to the war.
Tet is the Chinese New Year and the Communists chose this date as security was relaxed. Up until then the war had been largely fought in the countryside. Taking the war into the cities was a huge risk for the communist Vietcong and North Vietnamese. However their aim was to create an image in the minds of the American public, and administration, that the Americans were losing.
The strategy worked. Tet was a heavy military defeat for the communists. However the imagery portrayed shocked the American public. The media coverage, unprecedented before or since, showed the Vietcong occupying the American embassy in Saigon, the impact of napalm on innocent civilians, and the summary execution of a teenage Vietcong suspect, among many others.
Previous to Tet the American public had been told they were winning the war. However Tet showed an altogether different image and significantly increased opposition to the war.