Recidivism is the return to a behavior that is undesirable or illegal.
In the context of the law, recidivism is commonly used to discuss the possibility that a criminal or a group of criminals will return to performing criminal behavior in the future. Behavior that can qualify as recidivism includes arrests, incarcerations, drug relapses, etc.
The criminal justice system views recidivism as highly undesirable. This is because it means that punishments imposed by the legal system were not adequate to prevent the behavior from occurring again. So, efforts are frequently made to reduce recidivism rates. This could mean implementing certain programs in jails, or strategies for when inmates leave prison. Ultimately, the criminal justice system does not want repeat offenders, so actions/measures that reduce recidivism rates are generally viewed as positive.