The DMZ was a well defined border of the two countries at war, that could not be crossed at all times. But the North Vietnamese Army invaded the de-militarized zone, in hope to destroy the South Vietnamese resistance, and gain control over the south. This border between the two countries, provided a main battleground during the war, as well as many important military bases and support stations were located there. Some of the war's most violent and fiercest fighting occurred along this boundary dividing the two countries. The communist controlled North, lead by Ho Chi Minh, invaded the DMZ line, into South Vietnam, in hopes of gaining territory, and defeating South Vietnamese and American troops. There is a major importance of the de-militarized zone in Vietnam, because of it's separation between the two fighting countries, and being the area with the most deadly fighting in the war. This boundary between North and South Vietnam became the defining line between communism and democracy in southeast Asia.
Important Source: http://thevietnamwar.info/vietnamese-dmz/