Light is a transverse wave, meaning it oscillates perpendicular to the direction it's travelling — and unlike a wave on a rope, which you can shake up-and-down or side-to-side, ordinary light from the sun or a bulb is a mix of waves oscillating in every one of those perpendicular directions simultaneously. Polarisation is the process of restricting light to oscillating in just one of those directions, and once you understand that, an odd everyday effect — some car windshields going almost black through polarised sunglasses — stops being odd.
A filter that only lets one orientation of wave through
A polarising filter is built to absorb light oscillating in every direction except one, letting through only the component of the light wave aligned with the filter's own transmission axis. Unpolarised light passing through a single polarising filter loses roughly half its intensity — the half that happened to be oscillating in the blocked orientations — and emerges polarised, now oscillating only along the filter's axis. Polarised sunglasses use this deliberately: light reflecting off a horizontal surface like water or a road tends to become partially polarised in the horizontal direction, so sunglass lenses are oriented to block horizontally polarised light specifically, cutting reflected glare far more than they cut the rest of the scene.
Two filters at right angles block almost everything
Stack two polarising filters with their transmission axes at right angles to each other, and almost no light gets through either: the first filter polarises the light along its own axis, and the second filter, aligned perpendicular to that, has no matching component to transmit. Some car windshields and certain tinted or laminated glass have polarising properties built into their coating or the way stress is distributed in the glass, and when that polarisation happens to sit near ninety degrees to a pair of polarised sunglasses, the glass can appear to go dramatically darker, or show odd rainbow-coloured patterns, purely from the interaction of the two filters — an effect with nothing to do with the glass's actual tint.
What we're still unsure about
The physics of polarisation by absorption, reflection and scattering is thoroughly settled and precisely predictable — this isn't a live scientific controversy. What varies, and is harder to predict in any specific real-world case, is exactly how a particular pane of tempered or laminated automotive glass will interact with polarised light, since that depends on manufacturing details like how the glass was tempered and what coatings were applied, which aren't usually published by carmakers and can differ between models in ways that are only really discoverable by testing the glass itself.
This sits inside Polarisation of Light, one of eight topics in Waves & Optics, one of five domains in Physics, one of seventeen subjects the app can quiz you on.