As soon as you tick off a set, the rest timer starts running. You see how much time is left before the next set is due.
Why rest matters. With heavy sets, recovery between sets decides whether you manage the next one at the same weight. Resting too briefly unintentionally turns a strength workout into a conditioning workout; resting too long drags out your session without getting you anything more.
Rules of thumb. For heavy compound exercises -- squat, deadlift, bench press -- two to five minutes is usual. For isolation work, one to two minutes is often enough. In a superset you only rest after the last exercise of the round.
You’re not tied to it. The timer is a tool, not the police. If you feel ready, just start; if you need more, take more.
It keeps running in the background. If you put your phone down or switch to another app, the timer keeps counting.