A Billboard asset is a collection of pre-rendered images of a mesh. Use it with the Billboard Renderer to an object that is distant from the Camera at a low level of detail (LOD).
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. For more information, see the API reference for BillboardAsset.