In German, the inscription read "Arbeit macht frei." The English translation is "Work makes you free."
The inscription was probably intended to give the impression that those who worked hard would earn their freedom after a period of time as payment for their labor. Because the camp's activities were not publicly known, there was no way for persons being brought to Auschwitz to have any previous awareness of the camp or the consequences of arriving at that place.
The huge irony of the inscription is that the prisoners of Auschwitz gained their freedom by working themselves to death or by being sent directly to the crematorium. If prisoners were fit to work, they did so under unremitingly harsh conditions until they died or were physically unable to continue working. Prisoners who were unable to work, for whatever reason, were eliminated immediately. Either way, death did mean freedom from the extreme depravations of the concentration camp, but is not the definition of "free" that usually comes to mind.