Ορισμός και Σημασία του "ugly duckling"

What is the origin of the idiom "ugly ducking" and when to use it?

The phrase "Ugly duckling" comes from a classic children's story by Hans Christian Andersen called "The Ugly Duckling," in which a young bird grows into a beautiful swan. The idiom is often used to describe situations where someone or something undergoes a remarkable positive change.