Rehabilitation means that you lead a stable life and that you are unlikely to be involved in any further criminal activity. If you want to come to Canada, but you have committed or been convicted of a crime, you may apply for rehabilitation to enter Canada.
Requests for rehabilitation should be made at a visa office outside Canada. Such a request usually requires proof that at least five years have elapsed since the end of any sentence imposed (including any period of parole/probation), and that further criminal activity is unlikely.
