A repeat offender is a criminal that has already been convicted for a crime, and has now been caught breaking the law again.
E.G. a car thief steals a car and is caught and sent to prison. When the person is released they go and steal another car....this is a repeat offense, the person has not learned their lesson and this time the courts will pass a more severe sentence. In some countries if you are caught re offending 3 times you automatically get a life sentence on the third conviction.