The polariser is supposed to allow the transverse waves of the light to oscillate in one direction. To understand polarisation, think of a light beam as a train of electromagnetic waves. The electromagnetic forces making up these waves vibrate in a crosswise direction, perpendicular path of the beam. Waves that vibrate in this way are called transverse waves.
Ordinary light is composed of waves that vibrate in all directions perpendicular to the light beam.
But polarised light consists of orderly waves that vibrate in only one direction. If a polarising filter is placed in front of a torch light, all the light waves that pass through it vibrate in one transverse direction parallel to the transmission axis of the filter. The polarising filter absorbs some of the light that falls on it and transmit the rest. As a result, the brightness of the light beam is reduced. The filter does not change the propagation direction of the light beam.