The new Geo-fencing feature for ardupilot mega is ideal for beginners who want to learn how to fly.  The ardupilot mega geo-fencing feature allows the user to specify a GPS fence around a specific flying area, including a minimum and maximum altitude.  Think of it as a box the aircraft can fly in.  If your ardupilot mega powered UAV flies outside of this box, the autopilot will automatically take over and bring your Drone back to a pre defined loiter point and fly in a circle around that loiter point until you take over control again. 

This feature is ideal for beginners who are learning to fly as you if you fly too far away your UAV will turn around and come back home.  If you fly too low or too high, Ardupilot Mega will take over and make sure you do not crash.

Read more for some more info on how to use this new feature