It augments the protoc compiler so that it knows how to generate Go specific code for a given .proto file.

This plugin is provided google.golang.org/protobuf module, aka the second major revision of the Go protocol buffer API. Compared to the old protoc-gen-go plugin (in golang-goprotobuf-dev package) provided by github.com/golang/protobuf module, it doesn't support generating gRPC service definitions.