You'll probably find a better description in the Feynman Lectures, but I'll try to do it for you.
Plane polarized light can be represented as a superposition of two circularly polarized waves, a right handed and a left handed one. You can think of the waves as corkscrewing through space in a right or left handed fashion. When light passes through a material it is slowed down, the quantity that tells how much it is slowed down is called the "index of refraction". For opticaly active materials the index of refraction is different for right and left handed waves, so that when they are put together at the exit from the material to make a plane wave the plane of polarization will be rotated.