shown in other instead.
A billboard is a rectangular flat object that always faces the camera. Billboards are used instead of fully functional objects that are barely seen from far off and slightly change their position when the camera moves along them (e.g. clouds, stars, far away forests). Using billboards helps to simplify and speed up rendering.
For example, when you are adding a tree that is hardly seen from a distance, instead of creating a complex tree mesh just put an appropriate texture onto the alpha-tested billboard and it will make no visual difference at such a range.