How to Use The Rest Timer
The built-in rest timer shows you how long it's been since your last set, and when to do your next set.
To trigger the rest timer, tap any circle on the log workout screen. This means that whenever you complete a set, the rest-timer automatically starts for your next set. The timer is enabled by default so if you don't see it, it's probably disabled. Enable it in settings - timer - enable.
The timer rings every 1:30min, 3min or 5min depending on whether you succeeded/failed on the last set, and the sets/reps you're doing. The ringing means it's time to do your next set. If you struggled to get your reps on the last set (or failed) rest longer so you don't fail on the next set. For more info read this.
You can control the timer volume using the buttons on the side of your phone. Tap them on Android 7.x to get the main volume setting. Tap the down arrow on the right and two more sliders will show. The bottom slider controls notification volume. Use it to increase, decrease or mute the timer volume.
You can also put your phone in silent mode if you don't want to hear the timer but only want it to vibrate.