The International Numbers Station Monitoring Society is a group focused on researching numbers stations and the interest of shortwave radio.
A numbers station is a broadcast that sends out a message made from a list of numbers, Morse, or even polytones. Numbers stations are used to transmit coded messages to spies and is encrypted with a one-time pad. With the use of a one-time pad, there is no way to decode the message that is transmitted.
Only the person who has a copy of the one-time pad would be able to decode the message. This allows for complete anonymity which is why it is used for covert operations. With the message being sent through a SW broadcast the message can be heard from long distances and the identity of the people involved in the operations is kept secret.