To prepare yourself for your working sets, where you're lifting your target weight it's important to 'warm up'. The terminology can be confusing if you're new but Sets are how many times you perform an exercise for the desired amount of repetitions.
Prep sets are an extra set added to the start of an exercise. Except these sets half the target weight and double the reps. This is to pump blood into the muscles you're working in preparation for lifting heavier.
They're also ideal for practicing the movement pattern. This not only helps boost your performance and mind muscle connection but also can help with injury prevention.
They're often best added at the start of an exercise when you're changing the target muscle. For example, let's say you're just about to start targeting your chest having worked on a different muscle. Add a Prep Set to the first chest exercise to get ready. You can add prep sets to all your exercises but often times the first exercise of a specific muscle is enough.
Here's how to enable them.
Tap EDIT.
Tap the grey PREP SET icon to enable it. It will turn blue.
Tap DONE to save and return to your workout.