Fills the gap between sets with a countdown, a preview of what's next, and enough control to adjust it on the fly.
The header describes the UPCOMING set (the app has already moved the pointer forward), then the REST caption and countdown, then a NEXT preview of that set's weight and reps — skipped for cardio, which has its own screen instead.

The countdown is shared with your phone: if a workout is being logged from both devices, adjusting or skipping rest on one follows through to the other on its next update.
Related: "Logging a workout", "The set overview screen on your watch".