Golding first introduces the mirages in Chapter Four, "Painted Faces and Long Hair" in a sweeping, detailed depiction of the boys' beach at midday:
"Sometimes land loomed where there was no land and flicked out like a bubble as the children watched. Piggy discounted all this learnedly as a 'mirage'" (58).
It is really interesting that Golding provides this description of the mirages out on the reef in the same chapter that Jack discovers how to make a mask to disguise himself. He builds on the theme of appearances versus reality and the idea of deception. The mirage is a false image, projecting something that is not really there. In the same way, Jack will create a false image to disguise his appearance when he makes the mask out of clay. Golding's use of the mirage is a natural symbol for his larger theme of deception in Lord of the Flies.
The mirage is very significant as in the beginning, it fools the boys into thinking that the island is ideal; a paradise, even. But as they continue to live on the island, and are exposed to the practicalities of living away from society, from what they have known all their lives, the mirage slowly starts to re arrange itself until the boys see the island for what it is. This is of course very literal. But I also think that it has a wealth of meaning, metaphorically speaking. For example, the mirage hiding the island's true nature is very similar to the way the boys' civilised exteriors, in the beginning, hide their innate savagery.
Hope this helps :)