This function of ArduPlane allows you to set a minimum altitude for FBW-B mode (even if you don't have airspeed sensor) which your airplane will try to stay above.
When FBW Low Alt Limit is enabled, if your airplane goes lower than defined alt it will level up and climb back to the safe altitude. After it is reached you will regain full control.
This function will help those of us who doesn't have that much flying skills to fly safely. It is really useful for FPV flying as FBW-B mode provides easy control and the Low Altitude Limit will help you avoid crashing into the ground ;-)
Here is a video demo. APM is in FBW-B mode and I hold pitch all the way DOWN! until the very end of video. Minimum alt was set 30 meters above home altitude, so you can see ground.
Using your APM Mission Planner you will have to set parameter ALT_HOLD_FBWCM to the desired alt in centimeters! If it is 0 then it is turned off.
Desired altitude should be set very carefully. As we know our planes can't react immediately, so we have to choose altitude with this in mind. I recommend using 3000 (30 meters) for pretty flat terrain.
In order to land your plane you will need to disable this feature by changing your flightmode to anything other than FBW-B.