option go_package = "module_path/`"
When you go_package is set to a package, it will create the files at the directory.
It will create components such as types, marshalling/unmarshalling components, client/server interfaces
So in short: structs + boilerplate methods + service interfaces + client stubs.
So stub, in context here, represents a remote service locally, enables the client to call the services without handling network details.
“Just enough code that compiles and runs”