Prisoners are (in most cases) slaves of their own deeds. People are free to do whatever they wish. But some deeds are not good to others - like killing, stealing, etc. If they can't control themselves not to do those things (therefore "slaves of their own deeds"), then other people must take control over it and put such people into prison. By doing this, other people - especially those without control - are scared off similar deeds... A new chance is given to a prisoner when their sentence has finished and are free again...