https://en.wikipedia.org/wiki/Numbers_station

A numbers station is a shortwave radio station characterized by broadcasts of formatted numbers, which are believed to be addressed to intelligence officers operating in foreign countries.[1] Most identified stations use speech synthesis to vocalize numbers, although digital modes, such as Phase-shift keying and Frequency-shift keying as well as Morse code transmissions are not uncommon. Most stations have set time schedules, or schedule patterns; however, other stations appear to broadcast at random times. Stations may or may not have set frequencies in the HF band.

TDLR;
Global radio stations that broadcast on a regular schedule repeated numbers as part of a code.
Many have been going for years it's mostly accepted that spy agencies operate them.

Also of note:

North Korea revived number broadcasts in July 2016 after a hiatus of sixteen years, a move which some analysts regarded as psychological war.