The rest timer is there after every exercise (or round of exercises, if you've just done a superset) to give you an idea for the recommended amount of rest required between sets.
This has been programmed based on the style of training that you're supposed to do - so if you find yourself having to rest much longer every time, then perhaps drop the weight or the intensity. If you find yourself waiting around for the timer to finish consistently, then perhaps it's a chance to increase the weight and push yourself a bit further!
How can I skip the rest timer?
There is a ‘skip’ button in the top right hand corner of the rest page. Press it to skip the rest timer.
How can I extend the time on the rest timer?
There is a ‘+20s’ button on the rest page, in the middle of the rest timer. You can press it to add a little extra rest time. You cannot modify or set rest time for your exercises ahead of time.