An antagonist superset is a pairing of exercises with opposite movement patterns. Think of a classic push-pull or "front and back" superset.
One example would be doing dumbbell bench press, followed by a chest-supported row. The bench press would cover the "front" or push muscle group (chest), while the row would cover the "back" or pull muscle group (lats, etc.).
This type of training can add a new type of stimulus to your muscle fibers and pave the way for fresh growth.
Unlike agonist supersets, antagonist supersets can be performed more closely together in your training routine. For instance, you can do an upper-body antagonist superset on Monday and another on Thursday, while doing a lower body antagonist superset on Tuesday and another on Friday.