Who is Changelings from The Girl Who Circumnavigated Fairyland in a Ship of Her Own Making and what is their importance?

Changelings are human children taken from the human world by renegade fairies and brought to Fairyland, a fairy child having been substituted for them. Changelings are identified in Fairyland by the special shoes they wear, a rule imposed by the Marquess, who views them and their suffering as a key reason why the connection between Fairyland and the human world, as tenuous as it is, must be severed completely.