What does the RC_SIGN_IN means in googleplus login

RC_SIGN in is the request code you will assign for starting the new activity. this can be any number. When the user is done with the subsequent activity and returns, the system calls your activity’s onActivityResult() method. and that method will be like :

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
// Check which request we're responding to
    if (requestCode == RC_SIGN) {

    }
}

For more details you can see here.

For your requirement just assign any number to RC_SIGN.