Definition:
A Storyboard is a graphic organizer that provides the viewer with a high-level view of a project. Storyboard helps to ensure that everyone understand the goals of the project.
In agile software development, a storyboard can help developers quickly get a sense of what work still needs to be completed. Team keeps the storyboard up to date keeping, displaying all the completed work. This provides transparency, helps team to visualize sequence and interconnectedness of user stories. Storyboards can be physical or digital.
Further Reading:
Book: Agile Product Management with Scrum by Roman Pichler
Book: AGILE TESTING A Practical Guide for Testers and Agile Teams by Lisa Crispin
https://en.wikipedia.org/wiki/Storyboard