AutoTune attempts to tune the Stabilize P and Rate P and D terms that provide the highest response without significant overshoot. It does this by twitching the copter in the roll and pitch access which means that the copter needs to be basically flyable in AltHold mode before attempting to use AutoTune.
Note: This mode is just to optimize copter's flight, Erle-Robotics will deliver vehicle's parameters list to the customers.
If you wish to set-up autotune mode, follow the steps of this video tutorial.