Definition:
A process map is a tool to visually illustrate how the work flows, a visual representation of the steps needed to complete a piece of work. It is also a communication tool, a business planning tool and a tool to help manage the organization.
If you manage your work on a Kanban board, you probably started by creating a process map. Process maps can help you understand how work moves between phases before it is completed.
Further Reading:
Book: Kanban: Successful Evolutionary Change for Your Technology Business by David J Anderson
Kanban from the Inside by Mike Burro