The radio room is a room in the Rezidentura that is dedicated to the reception and transmission of encrypted radio signals and diplomatic cables. It is staffed by the radio operator.
Throughout The Americans there are multiple shots of the radio room. It appears to be a very small room filled with electronic equipment, a typewriter, and a small desk. It is barely big enough to accommodate four people.
Acting as a relatively (for the time) direct link to The Center, it would be the place where all highly-important, highly-classified communications are exchanged with Moscow.
In all instances of the radio room appearing on screen there are only ever two people in the room - the person dictating the message, and the radio room operator. The door remains locked when in use and it is only considered appropriate to disturb the radio room when in an emergency.
Radio Operator Edit
Being privy to all of the embassy’s classified communications would presumably require a high security clearance, making the role more than that of just a typist. They are required to encrypt and decrypt all messages sent or received via telegram, number stations etc.