Only transverse waves can be "polarized". (The word comes from "poles" - meaning, can have an orientation.)
The plane of polarization of a transverse wave is the plane in which the disturbance takes place. This is at right angles to the direction of the wave.
Unpolarized waves arrive with RANDOM directions of disturbance. Polarized waves can be formed from unpolarized waves by passing them through some polarizing process. A train of unpolarized waves in a rope can be polarized by passing them through a narrow physical gap.
We can use simple vector analysis to analyse the proportion of the amplitude passing through the polarizer.
Similarly, we can break the randomly arriving motions into two components mutually perpendicular to each other.As usual, with components, we can arbitrarily set up the axes. This is useful for analysis.
Light and all Electromagnetic radiation can be polarized, the direction of polarization being the Electric Field vector plane.
Polarization of light ( electromagnetic waves ) can be caused by a number of processes.
These first two are linked. Reflection and refraction occur due to electrons in the material responding to the incident Electric fields and reemitting the radiation back into the air and down into the material.
When the reflection is at 900 to the refraction, the transverse component of the Electric field lies along the path of the reflection. This would make it a longitudinal wave so clearly there is no transverse component in the reflection.
The incident angle at which this happens is called the Polarizing Angle or Brewster Angle. At other angles, the transverse response does not align with the direction of the reflection so a component is sent into the reflection as transverse.
Using Snell's Law and simple geometry gives a condition for Brewster's Angle, tanθ = n
(Note that the grating direction is exactly opposite to the rope gratings in the animation below. For microwaves at least, horizontal gratings transmit VERTICALLY polarized e/m radiation and vice versa.)