Rock salt was a type of salt. Believing salt to be the best protection there was, Martha Tyler used rock salt in her charms. She gave one to such charm to Leela, and another to her grandson, Jack Tyler. She also filled two cartridges with rock salt that Jack then used in his gun. The Fourth Doctor took the weapon from him in Fetch Priory when he was paralysed by the Fendahl and used the rock salt cartridges to kill a Fendahleen. He explained the salt's killing effect to Leela in saying that it "obviously affects the conductivity, ruins the overall electrical balance and prevents control of localised disruption to the osmotic pressures".
In need of more salt to combat the Fendahl, Martha asked Leela and Jack to give the Doctor her charms. He then asked her to collect salt, not just rock salt, for his use. The Doctor and Leela used vials of the salt to repel the Fendahl core while they took the skull, Eustace, out of the priory before the building was destroyed. (TV: Image of the Fendahl)