The word "shackle" has several meanings, depending on its context.
1) In a general sense, it refers to any type of restraint or bondage that limits one's freedom of movement or action. For example, someone could say "he was shackled by the chains around his ankles."
2) It can also refer to a specific type of metal cuff used for securing prisoners or detainees during transportation or imprisonment. In this context, it is often used in the plural form - "shackles" - and refers to the physical restraint itself. For example, someone could say "he was wearing leg irons with shackles."
3) Finally, "shackle" can also refer to a type of anchor that is used to secure boats or ships in port. In this context, it typically refers to the actual piece of equipment used for securing the vessel. For example, someone could say "the captain ordered the shackles dropped so we could sail away."