A radio transmitter was a type of electronic device used on Earth in the late 20th century. A transmitter could be programmed to transmit signals on a special, classified frequency. Cetacean biologists, like Doctor Gillian Taylor, would tag whales for the purpose of tracking their movement in the open ocean.
After Taylor gave the frequency to James T. Kirk in 1986, his crew were able to trace the humpback whales George and Gracie to the Bering Sea and save them from whale hunters. (Star Trek IV: The Voyage Home)