parser.add_argument( '-opt', type=str, required=True, help='Path to option YAML file.')

This line of code adds an argument to the command line parser. The argument is called "-opt" and it is of type string. It is also marked as required, so the user must provide a value for it. The help parameter provides a brief description of what this argument is for. Specifically, it is asking the user to provide the path to an option YAML file.