the short answer
A superset is two exercises performed back to back with little or no rest between them, counted as one set. You finish the first exercise, move straight into the second, then rest before repeating the pair. Supersets save time and raise workout density, and the two exercises can target opposing muscles, the same muscle, or unrelated groups depending on the goal.
This article is general training information for coaches, not medical advice - individual programming depends on a client's experience, goals, and recovery, so adjust the method to the person in front of you.