A Billboard Asset is a collection of pre-rendered images of a more complicated Mesh intended for use with the Billboard Renderer, in order to render an object at some distance from a Camera at a lower level of detail (LOD) to save on rendering time.
The most common way to generate a Billboard Asset is to create files in SpeedTree Modeler, and then import them into Unity. It is also possible to create your own Billboard Assets from script. See the API reference for BillboardAsset for further details.