What is a stub from the lightning grpc documentation?
It turns out, it is grpc nomenclature. According to the documentation at grpc.io:


  gRPC users typically call these APIs on the client side and implement
  the corresponding API on the server side.
  
  ...
  
  On the client side, the client has a local object known as stub (for
  some languages, the preferred term is client) that implements the same
  methods as the service.


Which pretty much means using grpc on the client side looks like you're interacting with an object that performs the operations locally - when in reality that object ( known as a stub ) is making calls to the server asking it to perform certain actions.