David Light describes Project Humminbird very well over on his Revit blog:
"In a nutshell Project Hummingbird is a set of Grasshopper components that facilitate the creation of Revit native geometry. This process utilizes a simplied pseudo-language in Excel to describe all aspects of the Revit BIM geometry. Translating Rhino primative geometry to Revit has been vastly simplied without the need for linking or reference objects. This allows for the downstream Revit model to be modified and adapted for the project duration. This process is not only limited to grasshopper but could also include read/write methods for Robot structural analysis, Processing, Cinema 4d and many others."
There's more information available on the Project Hummingbird website.
Further reading: Grasshopper-related add-ins.