A superset is a form of bodybuilding training in which you move quickly from one exercise to a separate exercise with in the same muscle group without taking a break for rest in between the two exercises.
Example:
Bench Press superset with Flat Bench Flyes
You would take you turn bench pressing and when the amount of reps to complete the sets are achieved you would then go straight into flat bench dumbbell flyes without much rest between exercises. Period of rest is usually just long enough to get situated for the new exercise. This is a Superset.