In Tom Wolfe's "The Right Stuff", what is the right stuff?

Of all the terms Wolfe coined to convey the experiences and codes of test pilots, "the right stuff" is the one that has most completely entered the language. In the world of the test pilot, a combination of skill, superstition, macho arrogance and reserve creates codes that are never put into words but are the standard by which men are judged. Of all the pilots, Chuck Yeager most embodies the combination of courage, skill and nonchalance which combine to form the indefinable "right stuff," and Wolfe devotes much of the book to scenes of Yeager in action, giving the reader a chance to understand it by example.