This analogy, similar to one attributed to Fresnel, provides a way to understand the phenomenon of polarization. Imagine a rope tied to a tree at one end with a person shaking it with a circular motion at the other end. The rope passes through three fences, with vertical bars in the first and the second fences and horizontal bars in the third, nearest the tree.
Moving the rope left to right creates waves that would be blocked by the fence because the horizontal gap is small. Thus, after the waves pass through the first fence, they move only vertically. After the waves pass through the third fence, the rope becomes straightened out.
This is a common phenomenon of light known as polarization; the first fence plays the role of a polarizer glass and the second one plays the role of a glass called the analyzer.