If something is unshackled, it's set free — anything confining it is loosened. When a guard removes the handcuffs from a prisoner's wrists, the prisoner is unshackled.
This adjective comes from shackle, a chain used to fasten someone's wrists or ankles together, and its Germanic root meaning "link of a chain." Remove someone's shackles and you get an unshackled person. You can use this word figuratively, too: "In the summertime, I feel unshackled from the constraints of school."