A Changeling was believed to be a fairy that had been left in place of a human baby stolen by the fairies. A fairy child may appear sickly and won't grow in size like a normal child, and may display intelligence far beyond their apparent years. They can also possess uncanny insight.
Most often, it is thought that the fairies exchanged the children. Occasionally an elderly fairy would be exchanged in the place of the human baby so the old fairy could live out its days in comfort, being coddled by its human parents.