Probation was a period of time in which a former offender's behavior was closely monitored.
In 2154, Hoshi Sato was allowed back into Starfleet on probation following her bad conduct discharge. She felt it was because Starfleet needed language specialists more than they needed to punish her. (ENT: "Observer Effect")
In 2364, following Q's release of USS Enterprise-D's crew, Jean-Luc Picard suggested they were only alive because they had been placed on probation, and a serious one at that. (TNG: "Encounter at Farpoint")