Supersets are performing two exercises back-to-back with little to no rest in-between, the time it takes you to get to the next machine or pick up the weights, once you have completed the set, which involved both exercises, then rest for 45-60 seconds to then repeat your sets.
There are typically two different ways of performing supersets.
The first is to use two exercises which use opposing muscle groups (non-competing supersets) like chest and back or biceps and triceps or to use two exercises which use mostly the same muscle groups (competing supersets) on chest, bench press and fly's, with the fly's you have taken the triceps out of action and can really bring the chest to fatigue.
Please when you have been training for around 4-6 months add these into your workouts mixing up with the other advanced systems.
Be sure to try, we rarely do supersets but are good if we need to fit more in a shorter workout, or up the heart rate and keep the workout pace faster.