Numbers stations, also known as spy stations, are curiosities that are usually found on shortwave radio. While there are lots of different types of numbers stations, they are almost always transmissions from secret government agencies as a way to securely communicate with their agents around the world.
Governments often transmit from central locations and broadcast all over the world. Agents can use an affordable and easy to acquire shortwave radio. These transmissions often contain a series of numbers and will often occur on a regular schedule.