Fitbit Premium is a paid-for subscription that will grant you access to workouts and mindfulness sessions. It will also open up certain insights: for example, accessing the Premium platform will show you your daily 'readiness' score, which tells you how ready your body is for a workout. You'll also get a more detailed insight into your sleep and stress scores.
Most new Fitbits will grant you a six-month free subscription to the platform, after which you'll have to pay $9.99 per month to access it.