The significance of the title of The Thorn Birds by Colleen McCullough is that it tells us about the book's main characters.
According to the old legend, the thorn bird seeks a thorn tree on which to impale itself from the very moment it is born. Once it has eventually found it and starts to impale itself on a thorn, the bird sings sweetly just before it dies.
In metaphorical terms, this is what characters like Meggie and Father Ralph de Bricassart do. They choose their own thorns at the cost of great emotional pain to themselves and others. In doing so, they realize that the best in life can only be obtained through suffering. And so, figuratively speaking, they impale themselves upon their thorns.
The downside, of course, is that they end up being hurt. But the upside is that, for an all-too-brief period of time, they experience the joy that comes through quite strongly in the beautiful song of the dying thorn bird.