Alma 52:8

and Moroni also sent orders unto him, that he should retain all the prisoners which fell into his hands; for as the Lamanites had taken many prisoners, that he should retain all the prisoners of the Lamanites, as a ransom for those which the Lamanites had taken.

Moroni also told him to keep all the prisoners he caught. Since the Lamanites had taken many prisoners, he should keep all the Lamanite prisoners. He could trade them for the prisoners the Lamanites had taken.

Moroni also sent him orders to hold onto all prisoners who fell into his hands. Since the Lamanites had taken many Nephite prisoners, Teancum should keep all Lamanite prisoners as ransom for their own.