Is the Chinese name of pl mirror in photography a polarizer? What's the use?

Yes, but there are two kinds of PL, one is called linear polarization, and the other is circular polarization that we often use. Polarization can eliminate the stray light on the surface of an object, for example, the reflection can be eliminated by the object behind the glass window, but it can't eliminate the reflection on the metal surface and mirror. Another advantage of polarization is that it can eliminate stray light in the air, so it is used when shooting the blue sky to make it bluer.