The person field allows you to link a card to one or more users in the collection team. When editing a card this field is presented as a set of checkboxes for each user in the collection.
This is useful for storing responsibilities in a card collection. For example: "Who is should complete this task card?"
The person field can be used when grouping a set of cards. In this way you can visually display which card is assigned to which person and easily make changes to this assignment by dragging & dropping cards.