QSL cards were postcards issued by shortwave radio stations to listeners in other countries. The broadcasters were usually foreign governments which wanted to propagate official viewpoints (i.e., propaganda) on a variety of subjects.
The shortwave listener would mail a report containing the details of a particular broadcast received (time, date, frequency, broadcast subject, etc.). If these details conformed with the station’s broadcast schedule, the shortwave station would mail back a confirmation (QSL Card) to the listener.
In this manner, the broadcaster obtained information on how successfully its signal was being received and the listener collected unique, colorful QSL Cards.