Warm-up sets are sets you perform at low weight prior to your work sets. Performing warm-up sets allow your body to gradually prepare for your work sets by loosening up the joints and increasing heart rate and blood circulation which increases blood flow to the muscles. This will ultimately reduce the risk of injury.
With Fitbod, you get a recommended weight scheme that builds up to your working weight for any exercise. This feature can be accessed within the Settings category of the Settings menu. Once ON, to see warm-up sets, first log an exercise so Fitbod has a baseline for your desired working weight or add a warm-up set by selecting the "..." More (bottom on exercise details) > Add Warmup Sets.