Fluke Networks explains fiber polarity

https://www.flukenetworks.com/blog/cabling-chronicles/b-c-s-fiber-polarity

Polarity defines direction of flow, such as the direction of a magnetic field or an electrical current. In fiber optics, it defines the direction that light signals travels through an optical fiber.

To properly send data via light signals, a fiber optic link’s transmit signal (Tx) at one end of the cable must match the corresponding receiver (Rx) at the other end.