In the context of US History courses, the term "second front" is most commonly used to refer to the Allies (mostly the British and the Americans) fighting against Germany in Western and Southern Europe.
For much of World War II, the only fighting that the German army was doing was on the Eastern Front where they fought the Soviets. The Soviets were having a very hard time resisting the Germans and they wanted the other Allies to open a second front in the west or south of Europe. The Soviets felt this would pull German troops away from the Eastern Front. The Western Allies were unwilling to do this for quite some time since they felt they were not ready to invade Europe.