Turtle mode is a possible rescue measure when the quadcopter has landed upside down. If you trigger the turtle mode, the motors spin backwards quickly so that the quadcopter can level itself again.
But beware! If the motors are blocked (e.g. by dirt or grass getting in through the upside down on the ground) this can quickly lead to the ESC overheating and dying.
Therefore, the turtle mode should only be used with caution in very short bursts.