A hall sensor (or Hall Effect Sensor to give it its proper name) is a semiconductor sensor that can detect the presence and magnitude of magnetic fields.
In the case of the LG machines they have a “Hall sensor assembly” on the direct drive motor, this has got 2 hall effect sensors within it and is used to detect the position and rotation of the motor so that they can drive the main motor windings in the correct order and accuratley sense the speed.
Hope that explains it a bit.
Jem