A superset pairs two or three exercises so you alternate between them with little or no rest in between, instead of finishing all of one before starting the next.
Build one from a routine's exercise list: swipe (or, on Windows, use the row buttons) and tap Group ⧉ to pair an exercise with the one right after it — groups can hold up to three exercises, and they must sit next to each other in the list. Ungroup breaks a member back out on its own. Cardio exercises can't join a superset. Reordering the list never leaves a broken group behind — if a drag or move would split a group apart, the app quietly ungroups it rather than saving something that no longer makes sense.
During a workout, a superset's exercises share one card that steps through the group in order — log a set on the first member and you're handed straight to the next one with a brief "no rest" note instead of the usual rest countdown, then on to the next, until the round is complete and a normal rest timer starts before the next round. The header counts the group as "Round 2 of 3" rather than a plain set count while you're alternating through it. A superset also counts as just one exercise in your overall "Exercise n of X" count, whichever exercise the card is currently showing.
If one exercise in the group has fewer sets than its partners, it simply finishes early and drops out of the rotation — from then on the remaining exercise runs solo with its normal per-set rest, same as if it were never grouped.
Related: "Routine detail", "Logging a workout"