A changeling is a substitute that the faeries put in the place of a stolen human child. First, they persuade a child to come away with them. Then they replace the stolen child with a sickly substitute that will sicken and fade away. It will die, but the child will be safe in fairyland.
In the poem called The Stolen Child, William Butler Yeats describes the coaxing away of a human child. The child is not actually stolen, but is seduced away to fairyland. The fairies are so poetic, and the world they reveal so enchanting, that the child forgets his family and home, and goes with them. The human world, they tell him, is full of tears, but their world is playful and green.
The poem takes the form of a tour of the country around the Irish town of Sligo, where Yeats spent his youth in his mother's hometown. The journey can actually be followed on a map. First the fairies and the child visit Lough Gill, where Sleuth Wood dips into it. Only some can see the underwater city here. The faeries refer to a leafy island in the lake, but there are about twenty, one of which is the famous Lake Isle of Innisfree. The faeries offer the child sweet fruit, and freedom from care.
Then the fairies describe the beach at Rosses Point, with its wide sands where they will dance all night, chasing the foaming waves in and out. Next, they lead him to a 50-foot waterfall in narrow Glen-Car, not to catch trout but to tease them, whispering to them from the branches of ferns that arch out over the water. Some say the child drowns here, or in the lake where the stream carries him. Perhaps the faeries take him to a barrow on top of the hill.
In any case, they succeed in their persuasion. The solemn child turns away from the warm and comfortable world of men, to join the faeries. His mother will cry for him, but he is too young to really understand that.
The poem was written in 1886, and published in 1889. Yeats was 21 when he wrote it, and at the beginning of his career. It celebrates the stories of Ireland that his mother loved. The images are consciously quaint. Yeats would later compile books of Irish fairy lore.
As he did though, his style grew grand and austere, moving away from the lush, folkloric description of this lovely early poem. Yeats may have loved Ireland in the abstract, and expressed its spirit more than well enough to win a Nobel Prize, but in this poem he loved the actual place in all its reality, and with all the intense enthusiasm of his youth.