Holiday in Cambodia is a song by Dead Kennedys which is about an American kid who expects to go off to college and thinks he knows everything about the ghetto (Braggin that you know how the n*****s feel cold and the slums got so much soul) and then gets drafted and sent to Vietnam. Pol Pot was the dictator of Cambodia during that time.
Pol Pot melted people, which is called a Biafra, so when they came out, they looked like jello; hence the lead singer's name Jello Biafra. While playing this song live, Jello would act like a dumb American kid who suddenly ends up in Vietnam and gets shot. (Thanks, Morgan - Huntington beach, CA, for above 2)
The line, You're a star-belly sneech is a reference to a Dr. Seuss character.