Gatsby's dreams are entirely that—dreams. In other words, they have no basis in reality. They are phantoms, fantasies, visions, and nothing more. They are about as unreal as it's possible to be, founded as they are on a fairy's wing. Though they may be secure on that fairy's wing, they have no anchor in the real world, the world in which Daisy Buchanan and the East Egg crowd live, move, and have their being.

Gatsby may have acquired phenomenal wealth; he may have a large mansion with lots of nice shirts in it; he may, in short, have achieved the American dream, but there's one dream he can never realize, and that's to be with Daisy Buchanan.

This is because Daisy, despite the brutishness and serial philandering of her husband, Tom, will never marry Jay. Gatsby thinks they had something special between them way back in the day in Kentucky. But in actual fact, this is just a fantasy on Jay's part; Daisy never felt about him the way he felt about her.

And although Daisy has an affair, although she tells Jay that she loves him, in actual fact, there's no chance that she's going to leave Tom. This is because, when it comes down to it, Jay is not from the same social background as the blue-blooded Daisy. He may be extremely rich, but Jay lacks the breeding that the likes of the Buchanans regard as so incredibly important. And so Gatsby's dream of being with Daisy is just that, a dream fastened securely to an angel's wing but to nowhere else.

This line from chapter 6 of The Great Gatsby shows that Jay Gatsby is the ultimate idealist. The contrast between the "rock" of the real world and the "fairy's wing" is meant to illustrate how Gatsby's life has been formed by his fantasies of wealth and class. As a younger man, he was not content with reality as it was and sought to redefine himself to the point of rewriting his own personal history. Even changing his name from James Gatz to Jay Gatsby is a part of this desire to change reality.

For Gatsby, the reality of the world is no concern. That Daisy has been married several years, that she has a child with her husband, and that she might even have cared for her husband at some point never occur to Gatsby, because they go against his romantic dreams. He continues to fight reality even after it is clear that Daisy is never going to leave Tom. His reluctance to even drain the pool once autumn comes is another extension of this trait: he holds onto youthful dreams even when it is proven they will go nowhere. And once Gatsby's dreams are shattered, he literally has nowhere else to go, making his death all the more tragic.

Jay Gatsby is a true romantic who believes that dreams and high ideals are essential elements that lead to success. His dreams signify the hope that he has to achieve his greatest aspiration: to once again be with his true love, Daisy Buchanan.

The metaphor is an apt description of the depth of Gatsby's dream. To him, the dream and reality are the same thing. The reference to a 'rock' indicates stability and strength - a solid foundation on which something can be built. To Jay, his dream is the foundation from which he can pursue and achieve his ideal. What makes the prospect more tantalizing is the fact that 'the rock of the world' is founded on a fantasy, 'a fairy's wing'. This is the nature of the true romantic - the dream becomes greater than reality and this provides the conviction that the dream is reality and not idle fancy - the lines are blurred. To a romantic such as Gatsby, there is no paradox.

In this sense then, Gatsby's dreams become a 'satisfactory hint' of this - that his dreams are real, that they have foundation and solidity, that they can be achieved. Gatsby has convinced himself that he will be able to win Daisy back and that she will want to be with him.

It is a pity then, that Gatsby has been so overwhelmed by his dream that he chooses to ignore the true reality: Daisy is married to an enormously wealthy man, she has a child, she is materialistic and manipulative, she has forsaken him, she is bored and seeks excitement and in some ways wants to get back at Tom; she is self-indulgent and spoiled and would never sacrifice her life of comfort and leisure to be with him.

Even when Daisy devastatingly tells him that he is 'asking too much', Jay persists and desperately clings to his dream. He drives her home and spends hours outside her mansion to ensure that she comes to no harm after she has accidentally killed Myrtle. It is this persistent hope that Nick admires so much but it is also this unrealistic and futile dream which, tragically, culminates in Gatsby's death. 

Gatsby's dreams of impressing Daisy, winning her back and returning to the love they had are all unrealistic--Daisy is married, and, while Daisy and Gatsby may entertain an affair for a brief period of time, Daisy is married to Tom.  Those bonds could not be broken easily by women in those days.

The quote from the narrator, Nick, is implying that things are always what we want them to be--what we may consider as a reality may be completely untrue.  Gatsby has been so delusioned by the idea of winning Daisy back that he concentrates on nothing but that, when, in reality, she won't leave Tom.  All the glamour and glitz of Daisy and Tom's life has overshadowed (originally) Nick's view of their marriage and what it's really like.  We think something may be secure (a life, a relationship, an economy), but the rock we thought it was built on was just resting on a fairy's wing.