Light from a regular source such as sunlight and lamplight is unpolarized. This means that the oscillations of the electric and magnetic waves that make up light occur in any plane. However, by using a polariser we can only allow a certain specific orientation of these EM waves through. Therefore, polarised light is defined as EM waves that oscillate in one plane.
Here is a link to a page with a simple diagram to demonstrate - https://physics.stackexchange.com/questions/231962/linear-polarized-3d-glasses-and-the-physical-shape-of-light-waves
- ↑ The Physics Classroom. Polarisation. year published= unknown. 23/10/2018; https://www.physicsclassroom.com/class/light/Lesson-1/Polarization