Light has an electromagnetic nature. The electromagnetic waves are transverse: the vectors E and B are perpendicular to each other and the oscillations are in the plane that is perpendicular to the direction of propagation.
The light waves emitted by the usual sources of light (lamps, for example) are unpolarized. This means that the oscillations of the vectors E and B occur in all possible transverse directions. Such light is called natural.
Some sources (such as lasers) emit polarized light. The oscillations of the electric and magnetic fields in polarized light do not occur in all directions, but only in the two perpendicular directions. Such light is called linearly polarized.
A variety of optical devices can turn unpolarized light into polarized light (a tourmaline crystal is a good example). The so-called polaroids have the same ability. A polaroid is a thin film of hepatite crystals. After passing through a polaroid, unpolarized light becomes linearly polarized. The direction of the electric-field vector in the transmitted wave is called the polarizing axis.
Polaroids are used for obtaining polarized light and analyzing it (polarizers and analyzers).
If natural light passes through two consequent polaroids, the intensity of transmitted light depends on an angle φ between the polarizing axes of the polaroids: