I have a sample helloworld.proto file and use Python. I am not getting what does this option keyword has to do in the compilation phase?
syntax = "proto3";
package services.helloworld;
option go_package = "github.com/xyz/api/go/services/helloworld";
To a python user? Probably not a lot. Options are parsed into the DSL object model (FileDescriptorSet), and can be used by whatever tool is processing the schema. The "go" processor presumably uses that option to determine a package/namespace/etc. The python processor, on the other hand, probably isn't remotely interested. There is no "py" equivalent, so I presume it isn't needed for python. As for what is does: from descriptor.proto:
// Sets the Go package where structs generated from this .proto will be
// placed. If omitted, the Go package will be derived from the following:
// - The basename of the package import path, if provided.
// - Otherwise, the package statement in the .proto file, if present.
// - Otherwise, the basename of the .proto file, without extension.
optional string go_package = 11;
Different options do different things; descriptor.proto is usually the best source for what inbuilt options exist (and what they do), however custom options can be defined by 3rd party tools.