Handcuffs are shackles. So are those leg irons some prisoners wear when they appear in court. In other words, a shackle is a restraint, either physical or psychological, that restricts movement.
We generally think of shackles as some sort of heavy metal cuff that is used to keep prisoners in check. But shackles don't have to be physical. Ignorance can be a shackle, so can an abusive family member, or the economy. In those cases, shackles are an external force that keep you from doing or being everything you want to. You can't see those shackles, but they can be every bit as confining as the metal cuffs.