Could you explain how the accelerometer calibration works

I have been following the space shooter tutorial and am confused with how the accelerometer calibration works . These are the 2 functions that was used

 void CalibrateAccelerometer()
     {
         Vector3 accelerationSnapShot = Input.acceleration;
         Quaternion rotateQuaternion = Quaternion.FromToRotation(new Vector3(0.0f, 0.0f, -1.0f), accelerationSnapShot);
         calibrationQuaternion = Quaternion.Inverse(rotateQuaternion);     
     }
     Vector3 FixAccleeration(Vector3 acceleration)
     {
         Vector3 fixedAcceleration = calibrationQuaternion * acceleration;
         return fixedAcceleration
     }

I am not able to understand how the accelerometer is calibrated with the above code .