A radio set was an electronic receiver that detected, demodulated and amplified transmitted signals.
In 2268, while talking to Bela Okmyx about the book known as Chicago Mobs of the Twenties, Captain James T. Kirk, with Spock on his side, asked Okmyx if the landing party left any other books, to which Okmyx replied "Sure. Some textbooks on how to make radio sets and stuff like that." (TOS: "A Piece of the Action")