The Smart Wake feature on Fitbit devices is an AI-based smart alarm. Once activated, Fitbit will attempt to find the best time to wake you up starting 30 minutes before the alarm time, based on your sleep stage.
With Smart Alarm, Fitbit helps you wake in light sleep stage. Waking up in your lighter sleep phases makes you feel energetic and refreshing throughout the day, as opposed to breaking sleep in the mid of a deep sleep.