“Die Gedanken sind frei” means “Thoughts are free.” This is a German song dating back to the 1780s.
It was often sung or recited by people in the German anti-Nazi resistance in World War II. In 1942, Sophie Scholl played the tune of this song on her flute, outside the prison where her father had been confined for insulting Hitler. She herself later joined the White Rose, a student resistance group. She would later be beheaded at the age of 21 for handing out anti-Nazi pamphlets, alongside her brother Hans and a third member of the White Rose.
I learned this from a recording of Pete Seeger; the words may be his translation.
This video was produced with the kind assistance of Dan Thurston.