Storyboard is a scene (e.g. "screens" or "views") management library that provides developers an easy way to create and transition between scene modules in a Corona SDK app. The purpose is very similar to the popular third-party Director Class module (by Ricardo Rauber).
Storyboard is the only official and supported solution to scene creation and management in Corona SDK, and is available for use starting with Corona build 2011.678.
To learn more about Storyboard, please see this blog post, and also see the Storyboard sample located in the /SampleCode/Interface/Storyboard folder within the Corona SDK download.
For an explanation of the different events associated with the Storyboard API, read: Storyboard Scene Events Explained
If you are a Director Class user, please see the Director to Storyboard "Transition" Guide.
Please see the discussion of these APIs in the Corona Forums.