A shield frequency refers to the settings that a particular starship has for their deflector shield system, which determines the frequency used by the shield emitters. If enemy forces obtain this frequency, it would allow certain types of adjustable weapons fire to be modified to pass through the shields as if they were not there. (Star Trek: Generations). The shield functionality can then be regained by remodulating the shields.
- This also explains how a ship can fire through its own shields. Their weapons would be set to their own shield frequency.