FDJ stands for Freie Deutsche Jugend and was the youth movement of the German Democratic Republic (DDR).
Although membership was voluntary, non-members often suffered pressure and discrimination and so around 80% of people between the ages of 14 and 25 were members.
Although closely associated with East Germany, the FDJ originally had branches in the western zones, before it was outlawed in the Federal German Republic (West Germany) in 1951. In fact, its earlier roots were not in Germany at all, rather in Prague, Paris and London in the 1930s.
Today there are still FDJ groups in Germany, although membership numbers are much lower than they were before reunification.
To hear a simple explanation and a short discussion in German, listen to the podcast:
(Press the “play” button to listen to the podcast)