This means that someone could be sentenced to slavery or indentured servitude. Federal prisons require able inmates to work; this is a form of indentured servitude, but it does not necessarily extend to state prisons (or municipal jails).
Slaves are property of others (until their death or a decision by their owner to free them), and can be traded or sold. Indentured servants are engaged for a specified period of time, usually to one specific employer.