To taste the plane of polarization of the microwave signal.
The polarization of an electromagnetic wave (such as a microwave) depends on the direction in which the electric field of the wave lies. The electric field varies within one plane only, so the wave is said to be plane polarized.
To specific cases of plane polarization are vertical plane polarization and horizontal plane polarization. If the direction of the electric field is vertical, then the wave is said to be vertically plane polarized. Conversely, if the direction of the electric field is horizontal, then the wave is said to be horizontally plane polarized.
Plane polarization is of particular interest because plane polarized waves are transmitted by microwave antennas and other types of radio antenna used in communication systems. The electric field generated by a transmitting antenna actually runs parallel to the antenna, while the magnetic field is at right angles to the antenna.
Figure: The polarization grille
If the grille is placed in the way of vertically polarized microwave signal, with the grille bars in the vertical position, then the signal is reflected. And if we place the grill in horizontal position, the signal will go through the grill.