A Superset (marked with a letter in the workouts) is built up of two exercises that you must perform back to back with no rest in between. Your only rest is to be taken once both exercises in the Superset have been completed! Burn!
For example: in a workout, you will see two exercises in a Superset – "A" Push-ups and "B" Sit ups, with 10 reps each. You would complete 10 Push Ups and then immediately move onto the Sit Ups for 10 reps (without resting). Once the Sit Ups are complete this is when you can rest before going back to the Push Ups once again!