This phrase was posted on the gates of the concentration/death camps.
“Arbeit macht frei” is the German version of Work makes free.
It is a twisted irony that the slave labor of millions of Jews, Jehovah's Witnesses, Gypsies, etc were worked literally to death in these camps. Their dead bodies then became hair for stuffed sofas, gold fillings from their teeth became riches for the SS, ashes of their remains became fertilzer for the fields. In some particularly bizarre cases, camp inmates with tattoos were killed just for the tattooed skin which was then tanned and made into lampshades.
It is a sad irony to those who perished under that phrase. “Arbeit macht frei”. I suppose it is only free if you consider death to be freedom.