A probationer is a person who is currently on probation. This is typically a person who was found guilty of committing a crime.
Probation is an alternative to a jail sentence; however, probationers are usually watched very closely for a certain amount of time as a trial period for their behavior.
If a probationer behaves well and does not commit any other crimes during probation, then they may be taken off probation after a certain amount of time. However, if they commit violations during that time, then they can face negative consequences, including jail time.
Probation is often given for less severe crimes and to first offenders. Repeat offenders are more likely to receive a prison sentence and not be offered probation.