All the fields that are marked as Header in the Layout Editor of the module will be shown in the Kanban View Card.
To include a field in the card shown in the Kanban view of Opportunities, please follow these steps
1. Click on Menu
2. Click on Opportunities
3. Click on Customize
4. Select Fields and Layout
5. Mark the field you wish to see in the Card of Kanban view as Header
6. Click on Save