Except for the colours of its spectrum, light also has a preferred direction. To understand what is meant by this preferred direction, it is important to know that light can be described as a fast moving wave. The oscillation frequency of the wave then determines the colour of the light beam.In the lefthand image, you see a wave, moving in the direction of the long arrow. The direction in which the wave oscillates is what we call the prefered- or polarization direction (you can see the direction of oscillation in motion if you click on the image). In the example of the lefthand image, the wave is polarized in the vertical direction. The image below show a wave that is polarized in the horizontal direction.
The video below show what polarisation is.