The 2010 smash-hit video game Call of Duty: Black Ops featured many of the conspiracy theories surrounding the Cold War. While some of them have been proven false, others are impossible to debunk — but a select few are very much true. One such example is the true-to-life way in which the protagonist receives orders throughout the campaign: through a "numbers station."
In the game, your character, Alex Mason, listens to a shortwave radio station transmitting from a boat off the coast of Cuba that intends to send a message to Soviet sleeper agents in the States. Unlike the more fantastical elements of the game, there is historical precedent for remote numbers stations being used by spy agencies of the time.
Before the era of radio encryption, anyone with a radio receiver could listen in on any conversation. Single-channel military radios operate much like the radio in your car, just at a much lower frequency — one that car radios can't receive. To make sure a secret message wasn't intercepted by a random person with a radio, agencies used cryptic codes. A well-known example of such secret speech is the American military's use of Code Talkers.
The other, equally ingenious method was the use of numbers stations. At a given moment and on a known frequency, a one-way message was sent. That message could be, as the name implies, just a string of numbers, either simply spoken or hidden within a specific song or Morse code. The listener would then use a cipher to translate what those numbers meant.
Someone could, for instance, turn on their car radio at exactly 12:34 PM and tune to a station that's normally just static and hear a person call off a string of numbers, which could then translate into something like, "continue the mission."
In the case of the video game Call of Duty: Black Ops, this method was used for espionage purposes. The radio station from which these messages were broadcast roamed the Gulf of Mexico, avoiding detection.
The use of open radio frequencies meant that more than one spy could listen in at the same time. Although never officially confirmed, many spy agencies from around the world have alluded to using them in such a manner.
Numbers stations are, allegedly, still in use. The confirmed Cuban numbers station, Atención, was at the center of an espionage case in the late 90s. Cryptic messages are still broadcast in Cuba at random times to this day.