Polarization of light

Light is a transverse wave, meaning its electric field oscillates perpendicular to the direction it's travelling — but perpendicular still leaves a full circle of possible directions to oscillate in, and ordinary light is a jumbled mix of all of them at once. Polarizing it means restricting that oscillation to just one direction.

Any transverse wave can be polarized in principle, not just light — the idea applies equally to a shaken rope, restricted to wiggling only up-and-down by a narrow slot. Longitudinal waves like sound can't be polarized at all, because there's only one direction available to them to begin with, along their direction of travel.

A polarizing filter works by absorbing light oscillating in every direction except the one aligned with its transmission axis, not by picking out a single colour — that's why stacking two polarizing filters at right angles to each other blocks light almost completely, regardless of the light's colour, while lining them up lets it through.