Auto-Level Mode (ACC or ANGLE) is where the copter's control system and sensors keep the copter level with reference to the direction of gravity. In this mode, if you do not give any input to the roll/pitch stick then the copter will remain level "on plane" at all times even if pushed by a gust of wind. Keep in mind though this is not position hold. Wind or other influence could still push the copter in any direction, but the copter will return to a level orientation. In contrast, a gust of wind in acro mode could cause the copter to bank or pitch. In MultiWii version 2.1 ACC mode was renamed to ANGLE mode.