RTH means “Return To Home”.
By design, when synchronized to GPS, Glonass and Galileo satellites and when short on power, your ANAFI series drone will always attempt to come back to its most recent take-off point (or to the pilot, or to a custom location – please also refer to additional details on advanced RT settings here), at a minimal height over this take-off point, which is configurable through FreeFlight 6 (between 20 meters and 100 meters) and is set by default at 30 meters.