The creditor, who is for some valid reason already in possession of property belonging to the debtor, is permitted to remain in possession of that property until the debt owing is paid.
This right, which is historically known as a 'lien', often arises by operation of law.
The creditor has the advantage of physical control of the chattel, but is not allowed to use or sell it. The debtor has the disadvantage of not being able to reclaim possession until the debt is paid.